Implement memlog on-disk handling (elastic#19408)

This change introduces the implementation of the on-disk state handling.
Tests are currently not included due to placeholders that still will
panic. The final state of the current implementation can be found here: https://github.com/urso/beats/tree/fb-input-v2-combined/libbeat/statestore/backend/memlog

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