rgw_admin: add rgw-admin reshard status command and initial blocking logic

rgw_admin: add rgw-admin reshard status command and initial blocking logic

A command that returns the reshard status field on the bucket index objects (all
shards).
Also, initial implementation of identifying and blocking while reshard in
progress.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>