rgwlc: remove bucket_lc_prepare, add backoff

rgwlc:  remove bucket_lc_prepare, add backoff

Remove now-unused RGWLC::bucket_lc_prepare.  Wrap serializer calls
in RGWLC::process(int index...) with simple backoff, limited to 5
retries.

In RGWLC::process(int index...), also open-coded the behavior of
RGWLC::bucket_lc_prepare(...), as the lock sharing between these
methods is error prone.  For now, that method exists, so that it can
be called from the single-bucket process.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>