Summary:
Re-committing.  Original: D6993857 revert: D7051998

Fixes for hazptr deletion on map destruction.
Several iteration cases don't properly release their hazard pointers.
This isn't disastrous, but means objects may stick around longer than necessary.
Adds tests and fixes for iteration to ensure we eagerly reset all hazard pointers.

Reviewed By: yfeldblum

Differential Revision: D7063727

fbshipit-source-id: 2ae32ead4965b37866096d2a8c6ea8c42c201335