Redis read-only replica support

[gthess]

This PR introduces a Redis replica server next to an Redis server that is assumed to be in read-only mode. If configured, Redis read commands will go there while write commands will go to the non-replica server. All connectivity options are duplicated for the replica server. Redis tests are not yet implemented.

[madnuttah]

Hi @gthess, does this imply this workaround for dockerized Unbound ain't necessary anymore then?

[gthess]

Hi @madnuttah, I am not sure what the workaround is in that link but I see no relation with this feature. This feature adds support for Unbound to issue all read commands to a Redis replica server, offloading most of the of the work from the main Redis server which will only be issued write commands. Does that answer your question?

[madnuttah]

Thanks for making it clear!