Refactor InternalPoolImpl so that NRE is not thrown.

https://github.com/couchbase/couchbase-net-client/pull/14
http://www.couchbase.com/communities/q-and-a/nullreferenceexception-memcachednodeinternalpoolimpl

NullReferenceException at MemcachedNode+InternalPoolImpl.<Dispose>

I recently updated to Couchbase Client Library .NET 1.3.4 and I've been experiencing daily incidences of the following NullReferenceException:

System.NullReferenceException
Stack:
at Enyim.Caching.Memcached.MemcachedNode+InternalPoolImpl.b__7(System.Object)
at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
at System.Threading.TimerQueueTimer.CallCallback()
at System.Threading.TimerQueueTimer.Fire()
at System.Threading.TimerQueue.FireNextTimers()

The exception is particularly problematic because it appears to happen within the Timer callback here: https://github.com/couchbase/couchbase-net-client/blob/master/src/Enyim.... which means that it bubbles up as an AppDomain.CurrentDomain.UnhandledException and terminates my process

I'm going to enable logging as described here (http://docs.couchbase.com/couchbase-sdk-net-1.3/#configuring-logging) in hopes that I get some more information to go on

• See more at: http://www.couchbase.com/communities/q-and-a/nullreferenceexception-memcachednodeinternalpoolimpl#sthash.x5T3rjeQ.dpuf