Back pressure fix for 3.7

[bentoi]

Here's a proposal for fixing back pressure on 3.7. The connection/thread pool implementation is now very close to the C++/IOCP implementation. The main difference is that IO operations are now started on the .NET thread pool to ensure it won't be interrupted if the thread exits. I'm undecided on whether or not it's a good idea to commit this large fix. We can use it for 3.8 however.

[bentoi]

As I understand it:

• we read (and write?) from a .NET threal pool thread with Task.Run() in startAsync

Reads and writes are started on the .NET thread pool and when completed, their completion callbacks are also executed on the .NET thread pool.

• upon completion, we queue a work item in the Ice thread pool

Right.

But then, how do we ensure we read only when the Ice thread pool is not exhausted? Is it because startAsync is called by the work item we queued?

If upon read completion the Ice thread pool is exhausted, the Ice thread pool queued work item won't execute until there's a free thread. So reads and writes won't progress.

[bentoi]

I've push additional fixes and add C++/Java tests.