Add key expiration (ttl)

[ashcrow]

A request came in asking to support ttl for keys. Both consultant etcd support ttl but in different ways. A single interface should be created so that switching is seamless.

[ashcrow]

• etcd: https://coreos.com/etcd/docs/2.0.8/api.html
• consul: Consul Session TTLs hashicorp/consul#524

[ashcrow]

Work started in https://github.com/ashcrow/consuloretcd/tree/ttl

Looked quickly at the consul implementation but it wasn't nearly as clear how to make a session expire a key.

[ashcrow]

Took a few times through the doc to realize that the session has to be made first and then the key must be created with the session. This seems to work. I'll try to implement it shortly.

curl example

The following expires the key after 30 seconds.

$ curl -H "Content-Type: application/json" -d '{"TTL": "30s", "Behavior": "delete"}' -X PUT http://127.0.0.1:8500/v1/session/create
{ "ID":"ad91aa2d-c562-182f-9938-7a913a32022e"}
$ curl -X PUT -d "test" http://127.0.0.1:8500/v1/kv/test2?acquire=ad91aa2d-c562-182f-9938-7a913a32022e

[ashcrow]

689cf0c implementes sessions for TTL in consul. Needs some clean up work but functionality should be there. Waiting on some user testing.

[ashcrow]

User testing came back positive.

[ashcrow]

Merged in #7