Deprecate/remove the overflowingExecutionContext

First commit — should be backported to the `2.3.x` branch so that we can fix #2947 and also do the deprecation for 2.3.

```
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.
```

Second commit — should *not* be backported because it removes something that was only deprecated in 2.3.

```
Remove overflowingExecutionContext

The overflowingExecutionContext was deprecated for Play 2.3 and can
be removed in Play 2.4.
```