ReplicatedPG: do not preserve op context during flush

Any information stashed in the OpContext may be obsolete by the time we
actually mark the object clean.  Instead, let the start_flush caller
clean up its OpContext and in try_flush_mark_clean we'll create a new
one.  The primary reason to keep the OpContext would have been locking,
but we can set the obc as blocking without holding an OpContext, and
that would allow trimming to happen in the mean time (which is good
since trim_object does not respect rw locks since it doesn't change user
visible state).  In try_flush_mark_clean, we requeue the fop->op along
with (but ahead of) the fop->dup_ops.

Fixes: #8068
Signed-off-by: Samuel Just <sam.just@inktank.com>