[2.2] Add a new store for counts

The added new store will provide statistics for the number of nodes (total and per label) and the number per relationship (total, per type, per start label, per end label and all other possible combinations).

New counts are persisted on disk every time the stores are flushed, in particular on log rotation.

Counts can be recovery as all the other store after a crash.

There is support for creating a new counts store when migrating from older versions.

The batch inserter (NOT the parallel batch importer yet) can upgrade counts when importing.

An interface for exposing counts to Cypher is stiil to be done.