Make RecursionGuard thread-safe,

by maintaining the stack of requests on a per-thread basis.
Made TerminatingWithPathSpecimenBuilder thread-safe,

by maintaining the stack of requests on a per-thread basis.
Added a test to help ensure Fixture remains thread-safe
Improved usage of ConcurrentDictionary by Thread,

because the GetOrAdd method easier to read and more idiomatic. Additionally, using the Thread instance as the dictionary key makes the intended usage more obvious.
Use ForceParallelism and ValueCollectingVisitor in fixture thread test.

Moved SpecimenWithEverything into AutoFixtureUnitTest project, and the IsPopulated() logic was also removed, and is now handled externally (by the test, and the ValueCollectingVisitor).