Fix unloadability races (#21024)

Fix unloadability races (#21024)

* Fix LoaderAllocator::AllocateHandle

When another thread wins the race in growing the handle table, the code
was not refreshing the slotsUsed local to the new up to date value. This
was leading to overwriting / reusing a live handle.
This change fixes it.

* Embed ThreadLocalBlock in Thread

Instead of allocating ThreadLocalBlock dynamically, embed it in the
Thread. That solves race issue between thread destruction and
LoaderAllocator destruction. The ThreadLocalBlock could have been
deleted during Thread shutdown while the LoaderAllocator's destruction
would be working with it.