Reduce visibility of RedisConn constants, renaming.

Nothing outside of RedisConn needs to know the precise implementation
of the Redis key names.
Adjust whitespace throughout.
Extract RedisConn payload to private method.
Remove RedisConn#redis,ImapConn#imap accessors.

There is no need for anything outside of these classes to use the
connections directly.
Rename core to base.
Rename read_mailboxes to queue_new_mailboxes_uids.
Merge branch 'refactor' into develop