Add (optional) sequentialId to StreamElement

[je-ik]

We need to add sequentialId to be able to identify ordering of updates within commit-log, even in the case when they have the same timestamp (or even timestamps swapped).

The sequentialId will be forced to be increasing (and therefore unique) at least within key+attribute pair. It might or might not be increasing over even larger group (e.g. for a whole entity key, for all updates within entity, or even globally).

The tuple (key, attribute, sequentialId) will be representing UUID in case when sequentialId will be part of StreamElement. We will implement this support for InMem and Kafka only (for now).

Part of #198