ReplicatedPG::do_op: check for blocked snapset obj

Otherwise, we might use an invalid snapset in find_object_context.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branches 'origin/wip-sam-testing2', 'origin/wip-7942', 'origin/wip-8020', 'upstream/wip-7891' and 'origin/wip-8091' into wip-sam-testing