MXS-1323: Fix crash on attempted retry of read

When a backend is waiting for a response but no statement is stored for
the session, the buffer where the stored statement is copied is not
modified. This means that it needs to be initialized to a NULL value.

Added a test that checks that the behavior works as expected even with
persistent connections. A second test reproduces the crash by executing
parallel SET commands while slaves are blocked.

There is still a behavioral problem in readwritesplit. If a session
command is being executed and it fails on a slave, an error is sent to the
client. In this case it would not be necessary to close the session if the
master is still alive.

maxscale-system-test/mariadb_nodes.h

@@ -16,6 +16,7 @@
 
 #include "mariadb_func.h"
 #include <errno.h>
+#include <string>
 
 /**
  * @brief A class to handle backend nodes
@@ -329,6 +330,7 @@ public:
      * @return Node id of the server or -1 on error
      */
     int get_server_id(int index);
+    std::string get_server_id_str(int index);
 
     /**
      * @brief Generate command line to execute command on the node via ssh