Refine database schema

[epicmonkey]

Redis list of improvements:

• convert UUIDs to Ints (so Redis uses Int Sets instead of Hash table)
• use Named Tuples for User object (i.e. replace username epicmonkey hashedPassword abc to epicmonkey abc and let backend map values as needed)
• replace ZSETs where applicable to LISTs/SETs
• introduce heads and tails (e.g. first 250 records are in head and others are in tail, head could be ZSET and tail could be LIST)

Postgres schema TBD