Deprecate and stop using the overflowingExecutionContext

Fixes #2947.

The overflowingExecutionContext is tricky to use safely so
this change removes all uses from Play. The object has been
deprecated for Play 2.3 and should be removed in Play 2.4.

In all cases the trampoline ExecutionContext should be used
instead since it is much safer and offers basically the same
performance.
Remove overflowingExecutionContext

The overflowingExecutionContext was deprecated for Play 2.3 and can
be removed in Play 2.4.
Merge pull request #2952 from richdougherty/no-overflow

Deprecate/remove the overflowingExecutionContext