Make Async.RunSynchronously more conservative about using current thread (reverting part of 11142)

[dsyme]

As described in #12637 (comment), part of #11142 created a problematic change in behaviour about when Async.RunSynchronously uses the current thread. Using the current thread is godo for performance and stacks but can cause deadlock if the current thread is relied on to process part of the async computation.

1. Prior to Await completed Tasks on the same thread. #11142 the current thread was used if SynchronizationContext is null (and no timeout specified)

2. After Await completed Tasks on the same thread. #11142 the current thread is used if Thread.CurrentThread.IsThreadPoolThread is true (and no timeout specified).

This change was not conservative - we now start on the current thread more often than we did before, specifically when we have non-null SynchronizationContext, yet CurrentThread.IsThreadPoolThread is true.

The idea of #11142 was not bad - it tries to make sure the computation always runs in the thread pool. However sometimes thread-pool threads have non-null SynchronizationContext.Current, and this is an indication that starting the async computation on the current thread is not acceptable.

As a conservative fix, we clarify the spec of RunSynchronously as follows:

The computation is started on the current thread if and only if all of the following hold: SynchronizationContext.Current is null, Thread.CurrentThread.IsThreadPoolThread is true and no timeout is specified. If any of these conditions are false it is started by queueing a new work item in the thread pool. This means that the computation will always be started on a thread-pool thread where the SynchronizationContext.Current is null.

This PR adjusts the code to match that.

Testing

Running without first adjusting tests to see if our existing tests clarify this.

[KevinRansom]

Release Notes:
Package: FSharp.Core
Description: Relax RunSynchronously reliance on current thread, eliminates a source of deadlocks
Issue: #2577
Executing a list of asynchronous computations hangs after updating to FSharp.Core 5.0.2

[dsyme]

I'll add a test for this