Implement "batch mode" in cli_mt

While testing an unrelated pull request, symat noticed that the cmd: "batch mode" argument (-c/--cmd with getopt_long) was ignored by cli_mt (as opposed to cli_st, which does the expected thing):

https://github.com/apache/zookeeper/pull/1131#issuecomment-561631843

It turns out that "batch mode" was never implemented for the THREADED case.

https://github.com/apache/zookeeper/commit/36ed46fc726fd#diff-419c86bcc09a6b28f27161879deed603R488-R492