Rearrange UUIDs before storing them (for efficiency)

[skalee]

In case of UUIDs version 1 (timestamp-based ones), a major performance boost can be achieved by rearranging bits. See https://www.percona.com/blog/2014/12/19/store-uuid-optimized-way/ for clarification and experimental proof.

This feature is highly desired, but will break compatibility with legacy versions of activeuuid. For this reason, it must be optional, and disabled by default.