modify zenhub healthcheck - use pb, rather than grep.

`(# of collectors on this hub) * (# of collector daemons) * (# of instances of each daemon)` is potentially a lot of new TCP connections to impose on the hub - and they could all be going over the WAN to the hub as well. While tracebacks in the hub are bad, I think untraceable connection starvation is a worse failure case.

What we actually care about here is the daemons own connection to the hub, right? So there shouldn't be a need for a brand new connection - it doesn't tell us what we want anyway.

I suggest adding some method for the collector daemon itself to output/store its connection state when it changes, and on maintenance cycle. The health check could then check the stored value for both contents and age to provide the status of the daemon's hub connection.

I could see an argument for this being a separate health check from `zenhub_answering`, but if we want to keep the TCP-connection test from all other containers I suggest we scale it back quite a bit - preferably by removing one or more of the scale factors rather than just dialing down the frequency.