rgw: perf -- remove bucket shards asynchronously rather than synchronously

rgw: perf -- remove bucket shards asynchronously rather than synchronously

We can now take advantage of the new asynchronous bucket shard removal
code and where we used to remove each shard synchronously now remove
them asynchronously. This would be a huge win when we have tens of
thousands of shards.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>
(cherry picked from commit cb0da45c5d6e4dff6514919b3b894df0eea7c1e4)

 Conflicts:
	src/rgw/rgw_rados.cc
conflicts with placement set and rgw rados refactor