rgw: reuse CURL* for keystone.

rgw: reuse CURL* for keystone.

When using keystone with https (the recommended setting),
it is inefficient to start up a new SSL connection for
each and every operation.  Keeping a CURL* structure around
should reduce the cost of doing this.  This logic tries
to do so, but it also tries to free them fairly aggressively
(5-10 seconds).  This should still greatly reduce load on
keystone at peak times while not tying up excess resources.

Fixes: https://tracker.ceph.com/issues/23162
Signed-off-by: Marcus Watts <mwatts@redhat.com>