When a controller returns a DeferredResult, the underlying async
request will eventually time out. Until now the default behavior was
to send a 503 (SERVICE_UNAVAILABLE). However, this is not desirable
in all cases. For example if waiting on an event, a timeout simply
means there is no new information to send.
To handle those cases a DeferredResult now accespts a timeout result
Object in its constructor. If the timeout occurs before the
DeferredResult is set, the timeout result provided to the constructor
is used instead.
Issue: SPR-8617
7ee821d3d1