Exposing control of the event-processor by SQL commands

When event-processor goes into the ERROR state, admin should run a global INVALIDATE METADATA to recover it. This is expensive and will impact all workloads using Impala.

It's proposed in this thread that we should expose a way to restart the event-processor if admins are sure that it's not stale (e.g. after manually refreshing some tables):
https://lists.apache.org/thread/5mw8jd5hgz7yycz9h2pxvqj101k0j47m

Both SQL commands and WebUI will be helpful.
CC myloginid@gmail.com hemanth619 VenuReddy maxwellguo