rgw: de-conflate shard_id and request_id in CLSRGWConcurrentIO

rgw: de-conflate shard_id and request_id in CLSRGWConcurrentIO

When using asynchronous (concurrent) IO for bucket index requests,
there are two int ids that are used that need to be kept separate --
shard id and request id. In many cases they're the same -- shard 0
gets request 0, and so forth.

But in preparation for re-requests, those ids can diverge, where
request 13 maps to shard 2. The existing code maintained the OIDs that
went with each request. This PR also maintains the shard id as
well. Documentation has been beefed up to help future developers
navigate this.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>
(cherry picked from commit 9606346592dfd6261aa2daa4cbec56f9a72c65fc)