Folsom metrics

Removes `ehmon` from logplex and replaces it with [`folsom`](https://github.com/boundary/folsom) and [`folsom_cowboy`](https://github.com/boundary/folsom_cowboy). The responsibility for getting the metrics out of logplex and the VM is now left up to an extern shh poller than can ping the `folsom_cowboy` poller as often as desired.

No live upgrade script is provided. This branch required updating cowboy, which meant adding cowlib and makes writing a manual live upgrade rather tedious and error prone.
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