Allow specifying the redis database index to be used

[acelaya]

Summary

Feature added via a contribution to shlink-common: shlinkio/shlink-common#150

It adds the ability to specify the redis database index to be used, via the connection URI path: http://redis_server:6379/5

Use case

For people with more than one database in the same server, this enables the option to choose which one to use. Without this, Shlink always uses database with index 0.