Feature summary (what you would like to be able to do and where):
A option that allows to specify the Redis database to use.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
Benefits (why should this be implemented?):
When running a wiki family, MAYBE spliting wikis into different Redis Databases will improve the access time. Also makes it possible to ask Redis to SAVE a single database, rather than all caches from all wikis.
When running many different softwares on a single Redis database, this helps avoid conflicts.
I think this is a better solution for the use cases mentioned in T371512.
This can be done by sending a SELECT command on all connections beginning.