rgw:gc: use XXHash for gc queue selection

rgw:gc: use XXHash for gc queue selection

We've observed significant imbalance in gc queue loading, most
likely due to poor ceph_str_hash_linux dispersion for common
key forms.

XXHash64 should be an effective and fast replacement for all
current RGW users of ceph_str_hash_linux (which also include
bucket-index sharding and bucket-lifecycle shard selection).

Unlike the other users, gc queue selection appears safe to
"just switch" as only defer-gc operations (which are non-critical
and of brief temporal locality) make any sort of rendezvous
on specific enqueued tags.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>