ReplicatedPG: in trim, grab w locks on obc and snapset_obc

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branches 'origin/wip-sam-testing2', 'origin/wip-7942', 'origin/wip-8020' and 'upstream/wip-7891' into wip-sam-testing