ReplicatedPG: make STARTSYNC a noop

Signed-off-by: Samuel Just <sam.just@inktank.com>
Fix replica lcod update, merge with big client io patch
PGBackend: merge with PGBackend transaction commit
ReplicatedBackend: merge with RPGTransaction commit
fix TRUNCATE case, merge with mod_desc filling in patch
remove Trans*t arg from trim, merge with ObjectModDesc interface patch
ObjectModDesc: make stashed, can_local_rollback private, merge
should_send_op patch: assert is_backfill_targets