rgw: RGWMetaSyncShardControlCR retries with backoff on all error codes

rgw: RGWMetaSyncShardControlCR retries with backoff on all error codes

RGWBackoffControlCR only treats EBUSY and EAGAIN as 'temporary' error
codes, with all other errors being fatal when exit_on_error is set

to RGWMetaSyncShardControlCR, a 'fatal' error means that no further sync
is possible on that shard until the gateway restarts

this changes RGWMetaSyncShardControlCR to set exit_on_error to false, so
that it will continue to retry with backoff no matter what error code it
gets

Fixes: http://tracker.ceph.com/issues/19019
Signed-off-by: Casey Bodley <cbodley@redhat.com>