threads: allocate on user's snapshotting heap

threads: allocate on user's snapshotting heap

The following comment is included in threads.h to explain this choice:

  Intentionally NOT allocated with MODELALLOC or SNAPSHOTALLOC. Threads
  should be allocated on the user's normal (snapshotting) heap to allow
  their allocation/deallocation to follow the same pattern as the rest
  of the backtracked/replayed program.

This fix solves problems seen in certain test programs regarding a
Thread being allocated in different locations for different executions.