Check for `background-processor` exit condition before+after sleep

Check for `background-processor` exit condition before+after sleep

In a synchronous `BackgroundProcessor`, the exit is done by setting
an atomic flag, which is most likely to happen while we're asleep.
Thus, we previously checked for the exit condition after the sleep
(and after we persisted the `ChannelManager`, if required, though
this is no longer required and dates back to when we didn't do a
re-persist after breaking out of the main loop).

For an async `background-processor`, this is also fine, however
because of the relatively longer sleep time, if the exit flag is
set via a sleep check returning true during event processing, we
may end up delaying exit rather substantially.

In order to avoid this, we simply check for the exit condition both
before and immediately after the sleep in `background-processor`.