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>
ReplicatedPG: do not queue the snap trimmer constantly

Previously, we continuously requeued the snap trimmer while in
TrimmingObjects.  This is not a good idea now that we try to
limit the number of snap trimming repops in flight and requeue
the snap trimmer directly as those repops complete.

Fixes: #9113
Backport: giant, dumpling, firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: dump snap_trimq on pg query

Signed-off-by: Samuel Just <sam.just@inktank.com>