Folsom metrics

Removes `ehmon` from logplex and replaces it with `folsom` and `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.