Initial support for pluggable storage backends

These changes add support for pluggable storage backends. Previously, the
storage layer only supported a single, built-in, in-memory storage backend.
Users integrating with OPA would write a shim layer that pushed data into this
backend so that the policy engine could evaluate over it. This approach is not
sufficient when the dataset is too large to replicate or the process of
replication would introduce race conditions into systems integrated with OPA.

With these changes, users can implement their own storage backends for OPA. The
storage backends are "mounted" into the root document at a particular
location. References to data under this location are then served by the
user's storage backend. This allows the storage backend to fetch data from the
source of truth thereby avoiding race conditions (when that is important).

These changes introduce new interfaces in the storage layer:

- Transaction interface
- Store interface (i.e., the pluggable backend)
- Storage object, represents the policy engine's storage layer
- Trigger interface