mesh-wide online awareness

In an effort to improve the reliability of `hq.report` there were some side-effects:

* whenever a lead scout comes online, `on.hq.online` is triggered across the whole mesh
* the command `hq.online` will return 1/0 on any scout if hq has been seen recently on the mesh
* the last `hq.report` will be persisted and resent until a lead scout acks it (good for triggers to make sure they're sync'd)