Memcached flush_all command pipelining problem

From Michal:

"memcached server module flush_all command cannot be in the middle of pipelined command input.
(this is probably tightly connected to JBPAPP-5439, but manifests itself in different scenario as well)

testcase:
$ echo -e "flush_all\r\nget a" | nc localhost 11211
SERVER_ERROR org.infinispan.server.core.ServerException: java.lang.NumberFormatException: For input string: "get"
expected output:
OK
END "