Add .NET Core 3.1 target that uses ASP.NET Core framework reference instead of NuGet package references

Add a missing dispose in managed client.

This reverts commit 6d0e2c2022.

There isn't any code in MQTTnet that actually uses a cancellation token in WaitAsync, so this is more of a preventative thing than a bug fix.  The original code just checks if the task was completed, not whether it was cancelled.  If the process is cancelled immediately before the call to WaitAsync, it'll return as normal (https://referencesource.microsoft.com/#mscorlib/system/threading/SemaphoreSlim.cs,612) rather than throw a cancellation exception.  This change will ensure we only return the releaser if the wait actually ran to completion rather than exited early due to cancellation.  I've tested this, and it properly throws a cancellation exception later on.

Sometimes, TryPublishQueuedMessageAsync would try to remove a message from the storage queue before PublishAsync added it to the storage queue, resulting in a message being stuck in the storage queue forever.  Switched the message queue lock to an async lock and synchronized the storage queue updates with the message queue updates.