feat: Add ability to set Redis database and prefix in config

[kentquirk]

Which problem is this PR solving?

In a blue/green deployment model, two refinery clusters might want to share the same instance of redis. It's also possible that multiple apps want to share one redis. This permits that by:

• allowing the redis "database" (an integer from 0-15) to be specified in the configuration
• allowing the redis "prefix" (a string used in the name of the keys stored in redis) to be set as well. It already existed but was hardcoded to "refinery".

Short description of the changes

• Add RedisDatabase and RedisPrefix to the PeerManagement section of config.
• Add tests
• Add documentation

Fixes #544