Improve correctness/completeness of library

[hartmut-co-uk]

Tasks

• wrap stores with MeteredKeyValueStore (builder)?
• provide timestampedKeyValueStore
• ? (TBC) logging / caching is always disabled (because it's not implemented to wrap store by CassandraStores...)
  • always disable logging + caching?

[hartmut-co-uk]

Analysed, nothing to do here.