Only perform causal reads on slaves

The causal read queries were performed also when the target server was the
master. The extra functionality of the causal reads is only needed on
slaves.

Adjusted the test case to require GTID replication.

maxscale-system-test/mxs1778_causal_reads.cpp

@@ -8,6 +8,7 @@
 
 int main(int argc, char** argv)
 {
+    Mariadb_nodes::require_gtid(true);
     TestConnections::require_repl_version("10.3.8");
     TestConnections test(argc, argv);
     const int N_QUERIES = 100;