[#2086] Fix race in Promise.onRedeem() that called "callback" twice

[davidcostanzo]

This fixes a subtle race condition that caused a variety of pathological behavior on a Play applications, particularly when a controller action invokes a job that finishes quickly.

1. await(Future) "returns" twice (in two different threads)
2. After await(Future), some controller action parameters are null.
3. In await(Future), there's a ConcurrentModificationException

The change includes a new regression test for the race.

[asolntsev]

@xael-fry Seems to be OK. Can be merged.