Sync slaves and writer thread in binlog_change_master

The test uses a separate writer thread to insert data into the
master. This thread must be halted before the blocking of the master
happens as the slaves must catch up. Once slaves have caught up and the
master is blocked, the writer thread can continue doing inserts.

At the end of the test slaves must also be synchronized before the
inserted data is validated. This prevents test failures due to slave lag.

maxscale-system-test/testconnections.cpp

@@ -819,7 +819,6 @@ int TestConnections::start_binlog(int m)
     binlog = open_conn_no_db(maxscales->binlog_port[m], maxscales->IP[m], repl->user_name, repl->password, ssl);
     execute_query(binlog, (char *) "stop slave");
     execute_query(binlog, (char *) "reset slave all");
-    execute_query(binlog, (char *) "reset master");
     mysql_close(binlog);
 
     tprintf("Stopping maxscale\n");