(wangle) fix race in Core::detachOne()

(wangle) fix race in Core::detachOne()

Summary:
In D1618240 I introduced a race condition in `detachOne()`, where `detached_` is incremented and then tested. If the promise and future are racing, they can both see `detached_ == 2` in the conditional, and then they'll both try to free the Core object. This fixes that.

It also fixes a related problem (which actually showed up more often with the test I wrote), where we transition into the Done state before setting the value, and then `maybeCallback()` observes the state is Done (because we're just reading an atomic, not grabbing the lock, which is intentional), tries to execute the callback, but `folly::Optional` throws an exception because the value hasn't been set yet (at least in debug it does). I should have listened to my gut and kept the state assignment after the transition action in the first place.

Test Plan: New unit test

Reviewed By: jsedgwick@fb.com

Subscribers: trunkagent, net-systems@, fugalh, exa, njormrod, folly-diffs@, mnd

FB internal diff: D1636490

Tasks: 5438209

Blame Revision: D1618240