Expose status of running queries via command or REST endpoint

[vcrfxia]

Today it's rather difficult for a user to know whether a continuous query is still processing data or has failed. Options to detect failure include:

• Noticing that the query is no longer processing data, by either monitoring the sink topic or via the runtime stats in the output of DESCRIBE EXTENDED on the sink stream/table

• Checking KSQL server logs and seeing errors indicating the Streams threads have died

• Observing that the JMX metric that exposes the status of the particular query has transitioned away from RUNNING

The last option isn't too bad, but ideally we'd also have a way for users to get query status more directly, either as part of EXPLAIN <query id> or a REST endpoint.

Note that this request is different from the existing /status/ endpoint which exposes status of the query in terms of whether KSQL has run the query or not. The request in this issue is about after KSQL has run the query, what is the status of the Streams application started by the continuous query.

[big-andy-coates]

Yeah, we should report the current running/not running state in DESCRIBE EXTENDED.