client_destroy destroys ZOOM_resultset (bug #3489).

ZOOM_resultset_destroy is now called in client_destroy. In earlier
1.4-releases of Pazpar2 it was called in client_set_connection.
It migth be good idea to add a ZOOM call to disassociate a
ZOOM_resultset from a ZOOM_connection to ensure no referral
to a ZOOM_connection in use by a different client (session).