System.AggregateException at Couchbase.Core.Buckets.CouchbaseRequestExecuter.SendWithRetryAsync

http://sdkbuilds.couchbase.com/job/win008-cbsdk-net-client2/290/consoleFull

I run the tests with async/await implementation that pointed in SDKQE-645

Jeff, could you please quickly review my cahnges. https://github.com/couchbase/sdkd-net/commit/e015f9b90604253086db8e7743ee8fe9ac0ffb35
Please note, that is separate branch for async/await implemenation, so we have 2 diff branches for now(with/out asinc)

at the end of rebalance there is an exception:

[617.60 DEBUG] (RebalanceWaiter sweepOnce:38) Rebalance Progress: 100%
[618.63 INFO] (RebalanceWaiter sweepOnce:33) Rebalance complete
[618.63 INFO] (Scenario run:97) CHANGE phase done
[618.63 INFO] (Scenario run:103) Starting REBOUND for 90 seconds
[618.86 INFO] (SDKD log:137) 
[618.87 INFO] (SDKD log:137) Unhandled Exception: System.AggregateException: One or more errors occurred. ---> System.NullReferenceException: Object reference not set to an instance of an object.
[618.87 INFO] (LineGobbler doFilter:115) +++ Following exception has internal ID: 1
[618.88 INFO] (SDKD log:137)  at Couchbase.Core.Buckets.CouchbaseRequestExecuter.SendWithRetryAsync[T](IOperation`1 operation) in c:\jenkins\workspace\win008-cbsdk-net-client2\couchbase-net-client\Src\Couchbase\Core\Buckets\CouchbaseRequestExecuter.cs:line 454
at Couchbase.CouchbaseBucket.UpsertAsync[T](String key, T value) in c:\jenkins\workspace\win008-cbsdk-net-client2\couchbase-net-client\Src\Couchbase\CouchbaseBucket.cs:line 2220
at Sdkd.Commands.SetCommand.<ExecuteSingle>d__0.MoveNext() in c:\jenkins\workspace\win008-cbsdk-net-client2\sdkd-net\src\Sdkd\Commands\SetCommand.cs:line 85
[618.88 INFO] (SDKD log:137)  — End of inner exception stack trace ---
[618.94 INFO] (SDKD log:137) [Sdkd.Result|Warn] Unrecognized return code 409 
[619.09 INFO] (LineGobbler doFilter:115) +++ Following exception has internal ID: 3
[619.09 INFO] (SDKD log:137)  at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
at System.Threading.Tasks.Task`1.get_Result()
at Sdkd.Commands.KVCommand.Run() in c:\jenkins\workspace\win008-cbsdk-net-client2\sdkd-net\src\Sdkd\Commands\KVCommand.cs:line 26
at SdkdConsole.Daemon.Handle.DispatchCommand(Request req, CouchbaseBucket cli) in c:\jenkins\workspace\win008-cbsdk-net-client2\sdkd-net\src\SdkdConsole\Daemon\Handle.cs:line 266
at SdkdConsole.Daemon.Handle.Run() in c:\jenkins\workspace\win008-cbsdk-net-client2\sdkd-net\src\SdkdConsole\Daemon\Handle.cs:line 146
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
[619.09 INFO] (SDKD log:137) 
[619.09 INFO] (SDKD log:137) Unhandled Exception: System.AggregateException: One or more errors occurred. ---> System.NullReferenceException: Object reference not set to an instance of an object.
[619.10 INFO] (SDKD log:137) +++ Received exception of ID 0
[619.10 INFO] (SDKD log:137)  — End of inner exception stack trace ---
[619.86 INFO] (SDKD log:137) +++ Received exception of ID 2
[619.86 INFO] (SDKD log:137) 
[619.87 INFO] (SDKD log:137) Unhandled Exception: System.AggregateException: One or more errors occurred. ---> System.NullReferenceException: Object reference not set to an instance of an object.
[619.87 INFO] (LineGobbler doFilter:115) +++ Following exception has internal ID: 5
[619.87 INFO] (SDKD log:137)  at Couchbase.Core.Buckets.CouchbaseRequestExecuter.SendWithRetryAsync[T](IOperation`1 operation) in c:\jenkins\workspace\win008-cbsdk-net-client2\couchbase-net-client\Src\Couchbase\Core\Buckets\CouchbaseRequestExecuter.cs:line 454
at Couchbase.CouchbaseBucket.GetAsync[T](String key) in c:\jenkins\workspace\win008-cbsdk-net-client2\couchbase-net-client\Src\Couchbase\CouchbaseBucket.cs:line 495
at Sdkd.Commands.GetCommand.<ExecuteSingle>d__0.MoveNext() in c:\jenkins\workspace\win008-cbsdk-net-client2\sdkd-net\src\Sdkd\Commands\GetCommand.cs:line 18
[619.87 INFO] (SDKD log:137)  — End of inner exception stack trace ---
[620.08 INFO] (SDKD log:137) +++ Flushing remaining exception trace buffer
[620.08 INFO] (SDKD log:137)  at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
at System.Threading.Tasks.Task`1.get_Result()
at Sdkd.Commands.KVCommand.Run() in c:\jenkins\workspace\win008-cbsdk-net-client2\sdkd-net\src\Sdkd\Commands\KVCommand.cs:line 26
at SdkdConsole.Daemon.Handle.DispatchCommand(Request req, CouchbaseBucket cli) in c:\jenkins\workspace\win008-cbsdk-net-client2\sdkd-net\src\SdkdConsole\Daemon\Handle.cs:line 266
at SdkdConsole.Daemon.Handle.Run() in c:\jenkins\workspace\win008-cbsdk-net-client2\sdkd-net\src\SdkdConsole\Daemon\Handle.cs:line 146
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()