InventoryStore improvements

[janm399]

There are two features that such cluster storage "things" should support:

• support for service discovery (the current Adapter code addresses this requirement)
• support for failure detection.

Obviously, the Adapter code could benefit from the addition of failure detection support. In etcd you currently have to implement this using sidekick services that maintain a TTL value on the key. In Consul (see http://consul.io), this is done using health checks (that a user pre-specifies).

//cc @carlpulley