rgw: use new Stopped state for special handling of 'bucket sync disable'

rgw: use new Stopped state for special handling of 'bucket sync disable'

some sync modules like pubsub can disable full sync, and only run
incremental sync. without full sync, 'bucket sync disable/enable' will
continuously restart incremental sync from the beginning of the bilog

this adds a new Stopped state for 'bucket sync disable', which leaves the
incremental marker position where it is (just past the SYNCSTOP entry).
this means that when sync is reenabled, it can continue sync from where
it left off

Fixes: https://tracker.ceph.com/issues/43768
Signed-off-by: Casey Bodley <cbodley@redhat.com>