Implement store operation that get/set key value pairs from
the in-memory store, and log update operations.

This change also include unit tests and store compliance tests.

The addition of the statestore package is split up into multiple
changeset to ease review. The final version of the package can be found
[here](https://github.com/urso/beats/tree/fb-input-v2-combined/libbeat/statestore).

Once finalized, the libbeat/statestore package contains:
- The statestore frontend and interface for use within Beats
- Interfaces for the store backend
- A common set of tests store backends need to support
- a storetest package for testing new features that require a store. The
  testing helpers use map[string]interface{} that can be initialized or
  queried after the test run for validation purposes.
- The default memlog backend + tests