Cache etcd data in memory for a few seconds to reduce etcd read throughput

[tarnfeld]

This is an experimental implementation of an in memory read-through cache for etcd keys. We'll hold data in the cache for just a few seconds, but that might be enough to significantly reduce read throughput to the underlying etcd cluster.

I'd be interested in any thoughts you may have @orls.

• Need to add some concurrency unit tests
• The 5 second TTL is probably too long as a default
• Allow the cache TTL to be configurable via command line arguments