Moved the housekeeper heartbeat to a separate header for ease of integration.

This keeps the behavior the same but allows the heartbeat to be used as a crude timer
without including everything the housekeeper.h header includes.

Moved the protocol level updates on the DCBs last_read value into dcb_read.
This keeps the implementation nice and compact.

server/modules/protocol/mysql_backend.c

@@ -447,9 +447,7 @@ static int gw_read_backend_event(DCB *dcb) {
 
                 /* read available backend data */
                 rc = dcb_read(dcb, &read_buffer);
-                
-		dcb->last_read = hkheartbeat;
-	
+
                 if (rc < 0) 
                 {
                         GWBUF* errbuf;