MXS-2563: Add test case

Added a test case that reproduces the debug assertion with the old code
and verifies that it is fixed with the new one.

maxscale-system-test/cnf/maxscale.cnf.template.mxs2563_concurrent_slave_failure

@@ -0,0 +1,26 @@
+[maxscale]
+threads=###threads###
+log_info=1
+
+###server###
+
+[MariaDB-Monitor]
+type=monitor
+module=mariadbmon
+servers=###server_line###
+user=maxskysql
+password=skysql
+monitor_interval=1000
+
+[RW-Split-Router]
+type=service
+router=readwritesplit
+servers=###server_line###
+user=maxskysql
+password=skysql
+
+[RW-Split-Listener]
+type=listener
+service=RW-Split-Router
+protocol=MySQLClient
+port=4006