osd/: switch all users of PGTransaction to use the new structure

osd/: switch all users of PGTransaction to use the new structure

This patch removes ReplicatedBackend::PGTransaction and implemenations
and switches over all users.  Happily, do_osd_ops loses the mod_desc
cruft and OpContext::pending_attrs.  PGTransaction doesn't really
have a natural way to implement append, however.  In reality, I think
this is probably an improvement, but it does mean that copy_from's
final transaction is now filled in by a lambda rather than by
appending a transaction fragment.

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