Current implementation of heartbeat spawns an independent routine to call Ping periodically.

It would be better if the ping timer was reset every time a message was received - that way we wouldn't call Ping more frequently than we actually need to.

We might also want to serialize calls to Ping with other calls to the SignVote and SignProposal, so that they only happen from a single routine and the underlying RemoteSignerClient need not be thread safe.

That should help clarify what needs a mutex and why. See #3064 for more.

Also, see http://250bpm.com/blog:22 for more info on the subtleties of heartbeats.