Implement a storage interface

This PR adds a TsdbStore interface that for now is simply a copy of the interface of the used methods on HBaseClient. The thought process behind introducing this middle layer is three-fold:

1. There now is in fact support for pluggable data-stores. It just has to implement an interface that is very HBase specific. The changes in this PR does not change any functionality, just the infrastructure.
2. We can start pulling out HBase specific implementation details into this layer and come with smaller pull requests that can be examined in more detail and has less of an impact.
3. By merging this into the `next` branch we don't stray too far from that and have one huge PR that has to be integrated into changes to `next`.

Let me know what you think :)