ObjectCacher: always complete flush_set() callback

ObjectCacher: always complete flush_set() callback

This removes the last remnants of
b5e9995f59d363ba00d9cac413d9b754ee44e370. If there's nothing to flush,
immediately call the callback instead of deleting it. Callers were
assuming they were responsible for completing the callback whenever
flush_set() returned true, and always called complete(0) in this
case. Simplify the interface and just do this in flush_set(), so that
it always calls the callback.

Since C_GatherBuilder deletes its finisher if there are no subs,
only set its finisher when subs are present. This way we can still
call ->complete() for the callback.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>