ReplicatedPG: clean out completed trimmed objects as we go

Also, explicitely maintain a max number of concurrently trimming
objects.

Fixes: 9113
Backport: dumpling, firefly, giant
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branches 'somnathr/wip-obj-delete-fix', 'origin/wip-9574', 'upstream/wip-8629', 'upstream/wip-9545', 'upstream/wip-9487' and 'origin/wip-9113' into wip-sam-testing