rgw_file: add timed namespace invalidation

rgw_file: add timed namespace invalidation

With change, librgw/rgw_file consumers can provide an invalidation
callback, which is used by the library to invalidate directories
whose contents should be forgotten.

The existing RGWLib GC mechanism is being used to drive this.  New
configuration params have been added.  The main configurable is
rgw_nfs_namespace_expire_secs, the expire timeout.

Updated post Yehuda review.

Fixes: http://tracker.ceph.com/issues/18651
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>