Optimizing :enter - :exit pairs.

Doing so by not letting any events between XXX:enter and XXX:exit
events. This eliminates unnecessary permutations.

Example: 2 threads with 3 enter-exit pairs each would have produced 924
permutations before this, now only 20.

Bug: 120628042
Change-Id: Ia243d273a1d90202011679cc7520ea4c9e43918b
Tests: All tests that use race condition framework