Partial fix to bug #657, http://bugs.mariadb.com/show_bug.cgi?id=657

dcb.c:dcb_clone: set same close function than in the original DCB so that routing session will become closed when client DCB is closed.
session.h:introduce a new state which indicates that SESSION structure can be freed next, SESSION_STATE_TO_BE_FREED. State is needed to separate the state before and after memory free. Memory is freed in different situation depending on whether the session is parent or child session. Child sessions are freed in their parent.
Also introduce a new member in SESSION struct, ses_is_child, to indicate whether SESSION has a parent session which is responsible for releasing the memory of it.

session.c:session_unlink_dcb:when last session reference is removed set SESSION->state=SESSION_STATE_TO_BE_FREED
session.c:session_free:only free the memory if session is not child session. If it is child, session is left to be freed by the parent (in tee filter's freeSession, for example).

tee.c:newSession:mark branch session to child.
tee.c:freeSession:if child session memory is ready to be freed, free it.

mysql_mon.c:monitorMain:when backend server's status changes so that it is not running anymore or doesn't have any of the states - master, slave, ndb, joined - call for each DCB the callback-function DCB_REASON_NOT_RESPONDING if specified. Earlier callbacks were called if there was any change in the state of the server being monitored.

server/modules/protocol/mysql_backend.c

@@ -1510,7 +1510,14 @@ static GWBUF* process_response_data (
                 bool               succp;
                 
                 srvcmd = protocol_get_srv_command(p, false);
-                
+
+		LOGIF(LD, (skygw_log_write(
+			LOGFILE_DEBUG,
+			"%lu [process_response_data] Read command %s for DCB %p fd %d.",
+			pthread_self(),
+			STRPACKETTYPE(srvcmd),
+			dcb,
+			dcb->fd)));
                 /** 
                  * Read values from protocol structure, fails if values are 
                  * uninitialized.