SGF-328 - Add missing 'hostname-for-senders' attribute on the <gfe:gateway-receiver> element in the SDG XML namespace (XSD).
(cherry picked from commit c6c7b59773ec6ea8eccb0e04570519127552f76f)

Signed-off-by: John Blum <jblum@pivotal.io>
SGF-328 - Additional refactoring after the cherry-pick commit in revision 283a762; Changed GemFire GatewayReceiverFactory 'setHostnameForSenders' calls to 'setHostnameForClients' based on interface differences between GemFire 7 and GemFire 8.
SGF-330 - Add missing 'disk-synchronous' attribute to the <gfe:gateway-sender> element in the SDG XML namespace (XSD).