PCT-671: Support for explain

Alpha prototype, WIP
That's a good question. Actually maybe this flag is not needed here anyway.

@daniel-nichter What's the purpose of 'running' flag? And why we are protecting it with mutex? I'm not asking about above example (which is my WIP code :)) but in general like here: https://github.com/percona/percona-agent/blob/release/mm/manager.go#L57 or here: https://github.com/percona/percona-agent/blob/release/qan/manager.go#L45 and why there is none here: https://github.com/percona/percona-agent/blob/release/instance/manager.go#L30