Currently the BulkProcessor class uses a single scheduler to schedule
flushes and retries. Functionally these are very different concerns but
can result in a dead lock. Specifically, the single shared scheduler
can kick off a flush task, which only finishes it's task when the bulk
that is being flushed finishes. If (for what ever reason), any items in
that bulk fails it will (by default) schedule a retry. However, that retry
will never run it's task, since the flush task is consuming the 1 and
only thread available from the shared scheduler.

Since the BulkProcessor is mostly client based code, the client can
provide their own scheduler. As-is the scheduler would require
at minimum 2 worker threads to avoid the potential deadlock. Since the
number of threads is a configuration option in the scheduler, the code
can not enforce this 2 worker rule until runtime. For this reason this
commit splits the single task scheduler into 2 schedulers. This eliminates
the potential for the flush task to block the retry task and removes this
deadlock scenario.

This commit also deprecates the Java APIs that presume a single scheduler,
and updates any internal code to no longer use those APIs.

Fixes elastic#47599

Note - elastic#41451 fixed the general case where a bulk fails and is retried
that can result in a deadlock. This fix should address that case as well as
the case when a bulk failure *from the flush* needs to be retried.