'nodetool cleanup' must wait for termination of cleanup, however,
cleanup is handled asynchronously. To solve that, a mechanism is
added here to wait for termination of a cleanup. This mechanism is
about using promise to notificate waiter of cleanup completion.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <6dc0a39170f3f51487fb8858eb443573548d8bce.1455655016.git.raphaelsc@scylladb.com>