Make KV distribution more flexible

[t-erxleben]

In a short brainstorm session with @JBenda we thought of adding an extra byte to KV keys. That would enable for 8 flags to be set.
Flags could include something like placed_on_purpose s.t. it is possible to distinguish between automatic placement and explicit placement (j_kv_new_for_index()). With this extra information one could write a tool to migrate data when new KV-backend servers are added. Explicitly placed KVs would be safe and not moved.

Other use cases might include something like place_on_fastest_server (in whatever metric).

Another interesting thought would be to implement several distribution schemes for KVs (e.g. based on namespaces). The extra flags could be used to encode them as well (again for migration purposes).