Consul runner manages heartbeats given --heartbeat option

[ethanrowe]

Given a --heartbeat option, with a periodicity in seconds, the runner subcommand should issue heartbeats in parallel to the agent's running.

The agent should be monitored and the whole process should exit with the agent's exit code.

This will give us a single entry point for driving the whole utility as one unit.

[ethanrowe]

I'm using this via a -t or --ticks option rather than --heartbeat. But same thing.

This is working in testing on vagrant.