This commit fixes sporadic failures in galera_3nodes_sr.GCF-336
test. The following changes have been made here:

1) A small addition to the test itself which should make
   it more deterministic by waiting for non-primary state
   before COMMIT;
2) More careful handling of the wsrep_ready variable in
   the server code (it should always be protected with mutex).

No additional tests are required.