Porting wsrep_reject_queries from Percona XtraDB Cluster

Hello,

this is a feature request to port the following commit to MGC:
http://bazaar.launchpad.net/~raghavendra-prabhu/percona-xtradb-cluster/pxc-1179805/revision/442

https://launchpad.net/bugs/1179805
Bug1179805: wsrep variable to reject queries

This commit adds a variable wsrep_reject_queries which can take 3 values:

a) NONE - default - nothing is rejected.

b) ALL - all queries are rejected with 'Error 1047: Unknown command'.

c) ALL_KILL - all queries are rejected and existing ones are also
aborted without waiting.

Thanks in advance.