Implement etcd connection pooling

[JamesLaverack]

Currently when we connect to etcd from the cluster controller we create a new connection every time. We could save resources by keeping a pool of them available.

[munnerz]

Be careful when doing this as you'll need a reconciler of some description to detect when clients should no longer exist or need updating to include new peers, else you could leave dangling sockets/goroutines/resources 😄

[JamesLaverack]

Yeah with #76 this might get interesting.