SslConnection fails when two or more threads write using MuxIO

I believe this is a regression from NCBC-1260 - when SSL is enabled, a series of calls on multiple threads will cause a NotSupportedException to be thrown. The reason being that MuxIO shares the connection and SslStream is not thread safe.

The breaking test is here: https://github.com/couchbase/couchbase-net-client/blob/master/Src/Couchbase.IntegrationTests/CouchbaseBucket_Ssl_Tests.cs#L80-L97

2017-01-09 18:41:36,428 [23] DEBUG Couchbase.IO.Services.MultiplexingIOService - Endpoint: 10.142.160.101:11207 - 5e63123b-671a-40dd-b5ff-d0ab815890e2 System.AggregateException: One or more errors occurred. ---> System.NotSupportedException: The BeginWrite method cannot be called when another write operation is pending.
at System.Net.Security._SslStream.ProcessWrite(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
at System.Net.Security._SslStream.BeginWrite(Byte[] buffer, Int32 offset, Int32 count, AsyncCallback asyncCallback, Object asyncState)
at System.IO.Stream.<>c.<BeginEndWriteAsync>b__53_0(Stream stream, ReadWriteParameters args, AsyncCallback callback, Object state)
at System.Threading.Tasks.TaskFactory`1.FromAsyncTrim[TInstance,TArgs](TInstance thisRef, TArgs args, Func`5 beginMethod, Func`3 endMethod)
at System.IO.Stream.BeginEndWriteAsync(Byte[] buffer, Int32 offset, Int32 count)
at System.IO.Stream.WriteAsync(Byte[] buffer, Int32 offset, Int32 count, CancellationToken cancellationToken)
at Couchbase.IO.SslConnection.<SendAsync>d__7.MoveNext() in C:\Users\jmorris\repos\couchbase-net-client\Src\Couchbase\IO\SslConnection.cs:line 191
— End of inner exception stack trace —
at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
at System.Threading.Tasks.Task.Wait(CancellationToken cancellationToken)
at Couchbase.IO.SslConnection.Send(Byte[] buffer) in C:\Users\jmorris\repos\couchbase-net-client\Src\Couchbase\IO\SslConnection.cs:line 175
at Couchbase.IO.Services.MultiplexingIOService.Execute[T](IOperation`1 operation) in C:\Users\jmorris\repos\couchbase-net-client\Src\Couchbase\IO\Services\MultiplexingIOService.cs:line 103
---> (Inner Exception #0) System.NotSupportedException: The BeginWrite method cannot be called when another write operation is pending.
at System.Net.Security._SslStream.ProcessWrite(Byte[] buffer, Int32 offset, Int32 count, AsyncProtocolRequest asyncRequest)
at System.Net.Security._SslStream.BeginWrite(Byte[] buffer, Int32 offset, Int32 count, AsyncCallback asyncCallback, Object asyncState)
at System.IO.Stream.<>c.<BeginEndWriteAsync>b__53_0(Stream stream, ReadWriteParameters args, AsyncCallback callback, Object state)
at System.Threading.Tasks.TaskFactory`1.FromAsyncTrim[TInstance,TArgs](TInstance thisRef, TArgs args, Func`5 beginMethod, Func`3 endMethod)
at System.IO.Stream.BeginEndWriteAsync(Byte[] buffer, Int32 offset, Int32 count)
at System.IO.Stream.WriteAsync(Byte[] buffer, Int32 offset, Int32 count, CancellationToken cancellationToken)
at Couchbase.IO.SslConnection.<SendAsync>d__7.MoveNext() in C:\Users\jmorris\repos\couchbase-net-client\Src\Couchbase\IO\SslConnection.cs:line 191<---