This ticket is for using a redis instance for caching for rdbms and 
anydbm backends as well as session database.

issue2550721 has a patch for using memcached to provide extended
caching of database lookups (for rdbms only). This improves
performance and the lack of persistence is not an issue.

Something similar could be done with redis. An added advantage of redis 
is that it has persistence. So it could be used not only for primary
database caching (where we don't care about persistence) but also
as a session db. For the session db we need things like session tokens,
registration tokens and json one time use keys to be persistent
across restarts.

Deploying redis for both of these roles would allow a performance 
increase using only one extra redis process rather than having
both memcached and redis.

Session db performance is an issue reported in msg6486 of issue2551036
when recording rate limiting parameters for the REST interface.