rgw: track initiator of reshard queue entries

rgw: track initiator of reshard queue entries

The logic for managing the reshard queue (log) can vary depending on
whether the entry was added by an admin or by dynamic resharding. For
example, if it's a reshard reduction, dynamic resharding won't
overwrite the queue entry so as not to disrupt the reduction wait
period. On the other hand, and admin should be able to overwrite the
entry at will.

So we now track the initiator of each entry on the queue. This adds
another field to that at rest data structure, and it updates the logic
to make use of it.

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