rgw: "bucket check --fix" should delete damaged multipart uploads from bi

rgw: "bucket check --fix" should delete damaged multipart uploads from bi

As one of the steps in `radosgw-admin bucket check --fix ...` it looks
for bucket index entries for incomplete multipart uploads that do not
have a corresponding ".meta" entry in the same bucket index. It then
intends to delete those entries, however the function that it calls
to perform the bucket index deletions was flawed and did not direct
the removals to the appropriate shard(s), but instead a non-existant
oid.

This commit determines the appropriate shard for each of the entries
to be removed and asynchronously issues a librados call to
omap_rm_keys.

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