Introduces `storage.Backend`, which will serve as the interface between the
Resource Service and the underlying storage system (Raft today, but in the
future, who knows!).
The primary design goal of this interface is to keep its surface area small,
and push as much functionality as possible into the layers above, so that new
implementations can be added with little effort, and easily proven to be
correct. To that end, we also provide a suite of "conformance" tests that can
be run against a backend implementation to check it behaves correctly.
In this commit, we introduce an initial in-memory storage backend, which is
suitable for tests and when running Consul in development mode. This backend is
a thin wrapper around the `Store` type, which implements a resource database
using go-memdb and our internal pub/sub system. `Store` will also be used to
handle reads in our Raft backend, and in the future, used as a local cache for
external storage systems.
