Speed up EventCount, now 64-bit only

Speed up EventCount, now 64-bit only

Summary:
Don't use two atomic variables when only one will suffice (otherwise, the
increments in doNotify() and prepareWait() would require memory_model_seq_cst,
because they need to globally order a store before a load, and no weaker
model allows you to do that)

Of course, this makes this 64-bit only, but I don't care.

Test Plan: eventcount_test, tests outside of folly

Reviewed By: delong.j@fb.com

FB internal diff: D858963