rgw: allow CLSRGWConcurrentIO to handle "advancing" retries

rgw: allow CLSRGWConcurrentIO to handle "advancing" retries

When doing an asynchronous/concurrent bucket index operation against
multiple bucket index shards, a special error code is set aside to
indicate that an "advancing" retry of a/some shard(s) is necessary. In
that case another asynchronous call is made on the indicated shard(s)
from the client (i.e., CLSRGWConcurrentIO).  It is up to the subclass
of CLSRGWConcurrentIO to handle the retry such that it "advances" and
simply doesn't get stuck, looping forever.

The retry functionality only works when the "need_multiple_rounds"
functionality is not in use.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>