Fix crashing InMemory.FunctionalTests

[BrennanConroy]

InMemory.FunctionalTests has crashed before due to an exception in a background task. When the InMemoryTransportConnection is closed it queues a callback to the threadpool which sets a TaskCompletionSource. If the callback is queued more than once the TCS will be set multiple times which is an error and exceptions in threadpool items will crash the process.

Simple fix is to avoid queuing the threapool item multiple times by making our closed bool set/check an atomic operation.

One test that would hit this is

aspnetcore/src/Servers/Kestrel/test/InMemory.FunctionalTests/ConnectionLimitTests.cs

Line 27 in f655a4a

public async Task ResetsCountWhenConnectionClosed()

where it would dispose the connection from test code, while in parallel the app code would start cleaning up which would call Abort.