MXS-1516: Validate the connection on each query

A subset of the checks done at connection creation time need to be done at
query routing time. This guarantees that the connection is closed if the
server no longer qualifies as a valid candidate.

Added teset case that checks that a change in the replication topology
correctly breaks the connection.

maxscale-system-test/mxs1516.cpp

@@ -0,0 +1,26 @@
+/**
+ * MXS-1516: existing connection don't change routing, even if master switched
+ *
+ * https://jira.mariadb.org/browse/MXS-1516
+ */
+
+#include "testconnections.h"
+
+int main(int argc, char** argv)
+{
+    TestConnections test(argc, argv);
+
+    test.connect_maxscale();
+    test.try_query(test.conn_master, "SELECT 1");
+
+    // Change master mid-session
+    test.repl->connect();
+    test.repl->change_master(1, 0);
+
+    test.add_result(execute_query_silent(test.conn_master, "SELECT 1") == 0, "Query should fail");
+
+    // Change the master back to the original one
+    test.repl->change_master(0, 1);
+
+    return test.global_result;
+}