rgw: bucket sync transitions back to StateInit on OP_SYNCSTOP

rgw: bucket sync transitions back to StateInit on OP_SYNCSTOP

the handling for OP_SYNCSTOP->OP_RESYNC is incorrect because it's not
safe to continue incremental sync after SYNCSTOP. any changes between
SYNCSTOP and RESYNC will not be written to the bilog, so incremental
sync cannot sync them

SYNCSTOP now transitions back to StateInit, where
RGWInitBucketShardSyncStatusCoroutine will query the remote bilog info
to see if it's still disabled. if so, it deletes the sync status object
and finishes. otherwise, StateInit will transition back to StateFull to
resync the bucket

Fixes: http://tracker.ceph.com/issues/26895
Signed-off-by: Casey Bodley <cbodley@redhat.com>