Replace monitor.go with a leased key and watcher

[mercul3s]

The current monitor is incompatible with a clustered sensu backend as the monitors are implemented in an in-memory map. We should instead use a key in etcd with a lease for each monitor, and a watcher for that key's expiration/deletion. We'll need to update the monitor implementation (replace the in-memory map) in keepalived and eventd once this work is done.

[grepory]

While we are at it, can we make this work with clustered backends?

[mercul3s]

@grepory that's the intent!