MXS-1275: Add client protocol data to MXS_SESSION

The current sql mode will be tracked in that variable. Since one
thread will handle multiple sessions, we need to track the sql mode
of a session, so that the sql mode of the query classifier can be
set whenever we handle a request of a particular session.

As the sql-mode will be managed by the client protocol, the data
can be private to it as well.

server/core/maxscale/session.h

@@ -27,7 +27,7 @@ MXS_BEGIN_DECLS
 
 #define SESSION_INIT {.ses_chk_top = CHK_NUM_SESSION, \
     .stats = SESSION_STATS_INIT, .head = MXS_DOWNSTREAM_INIT, .tail = MXS_UPSTREAM_INIT, \
-    .state = SESSION_STATE_ALLOC, .ses_chk_tail = CHK_NUM_SESSION}
+    .state = SESSION_STATE_ALLOC, .client_protocol_data = 0, .ses_chk_tail = CHK_NUM_SESSION}
 
 #define SESSION_PROTOCOL(x, type)       DCB_PROTOCOL((x)->client_dcb, type)