readwritesplit.c: prevent switching the master during session. Added logging to cases where master has changed.

Moved DCB's member errhandle_called behing DEBUG flags to Release build. It shows if handleError is called for a DCB and makes it possible to avoid redundant calls.

server/core/dcb.c

@@ -121,8 +121,8 @@ DCB	*rval;
 #if defined(SS_DEBUG)
         rval->dcb_chk_top = CHK_NUM_DCB;
         rval->dcb_chk_tail = CHK_NUM_DCB;
-        rval->dcb_errhandle_called = false;
 #endif
+	rval->dcb_errhandle_called = false;
         rval->dcb_role = role;
         spinlock_init(&rval->dcb_initlock);
 	spinlock_init(&rval->writeqlock);