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 pull request #2604 from athanatos/wip-9113

ReplicatedPG: clean out completed trimmed objects as we go

Reviewed-by: Sage Weil <sage@redhat.com>