osd/: replace simple_repop_.* with simple_opc_.*

osd/: replace simple_repop_.* with simple_opc_.*

This way, we don't expose the RepGather structure
up into the users (which pretty much exclusively
use the repop->ctx member anyway).  This will pave
the way to removing RepGather::ctx.

Part of this involves generalizing the repop callback
members (queue_snap_trimmer and on_applied) to
on_applied, on_committed, on_success, and on_finish
on both OpContext and RepGather.

Signed-off-by: Samuel Just <sjust@redhat.com>