Update to use new multi rexi_server protocol
Remember the on-disk order of replicas
Choose ushards according to persistent record

The order of nodes in the by_range section of "dbs" documents is now
promoted to the principal order for ushards. Ushards still accounts
for Liveness, selecting the first live replica and still supports
Spread by rotating this list using the CRC32 of the database name
(since many databases will have the same layout).
Add upgrade-safe shard access module