izettle-cassandra: Adds sequence generator.

This adds a generic sequence generator with all sequences persisted in Cassandra. This enables us to generate sequence numbers in a RDMS-style, event though we have a fully distributed system.

I have tested this running 20 threads in parallel on my local machine. We could perform tests in larger scale on a multi-node cluster if we feel that we need that confidence.

Ping @nzroller @oskarwiksten