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/cnf/maxscale.cnf.template.mxs1323

@@ -0,0 +1,51 @@
+[maxscale]
+threads=###threads###
+log_info=1
+
+[MySQL Monitor]
+type=monitor
+module=mysqlmon
+###repl51###
+servers= server1,server2
+user=maxskysql
+passwd= skysql
+monitor_interval=500
+
+[RW Split Router]
+type=service
+router= readwritesplit
+servers=server1,server2
+user=maxskysql
+passwd=skysql
+
+[RW Split Listener]
+type=listener
+service=RW Split Router
+protocol=MySQLClient
+port=4006
+
+[CLI]
+type=service
+router=cli
+
+[CLI Listener]
+type=listener
+service=CLI
+protocol=maxscaled
+socket=default
+
+[server1]
+type=server
+address=###node_server_IP_1###
+port=###node_server_port_1###
+protocol=MySQLBackend
+persistpoolmax=10
+persistmaxtime=300
+
+[server2]
+type=server
+address=###node_server_IP_2###
+port=###node_server_port_2###
+protocol=MySQLBackend
+persistpoolmax=10
+persistmaxtime=300