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/core/dcb.c

@@ -70,6 +70,7 @@
 #include <skygw_utils.h>
 #include <log_manager.h>
 #include <hashtable.h>
+#include <hk_heartbeat.h>
 
 /** Defined in log_manager.cc */
 extern int            lm_enabled_logfiles_bitmask;
@@ -816,6 +817,9 @@ int dcb_read(
                         n = 0;
                         goto return_n;
                 }
+
+		dcb->last_read = hkheartbeat;
+
                 bufsize = MIN(b, MAX_BUFFER_SIZE);
                 
                 if ((buffer = gwbuf_alloc(bufsize)) == NULL)