cls/rgw: adding an entry to reshard queue has O_CREAT option

cls/rgw: adding an entry to reshard queue has O_CREAT option

Adds the ability to prevent overwriting a reshard queue (log) entry
for a given bucket with a newer entry. This adds a flag to the op, so
it will either CREATE or make no changes. If an entry already exists
when this flag is set, -EEXIST will be returned.

This is a preparatory step to adding shard reduction to dynamic
resharding.

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