Deprecate JmxTool?

According to git-blame, JmxTool has been around since October 2011. We use it in system tests, but we are thinking it might be best to replace it: https://issues.apache.org/jira/browse/KAFKA-5612

When making modifications for system tests, we've had to take into account compatibility because this tool is technically included in our distribution and, perhaps unintentionally, a public utility.

We know that "real" tools for JMX, like jmxtrans, are more commonly used, but we don't know who might be using JmxTool simply because it ships with Kafka. That said, it also isn't documented in the Kafka documentation, so you probably have to dig around to find it.

Hopefully we can deprecate this and eventually move it either to a jar that is only used for system tests, or even better just remove it entirely. To do any of this, we'd probably need to do at least a cursory survey of the community to get a feel for usage level.