Adds etcache which caches requests to etcd.

This is the first stab at a fix for #8.
Further integrating etcache.

It now runs without crashing... but it hangs instead.
Etccache seems to be working.

The system seems to be able to push through decent size workloads.
Merge pull request #10 from pachyderm-io/etcd_cache

Etcd cache