Enable the usage of database replication

[jonasraoni]

Problem

In order to improve the performance of large installations, a database replication system might be very handy, but it's currently not possible to use it with our applications, due to the lack of read-only database setup.

Solution

We need to introduce a replication configuration for the database, and allow the user to setup addresses for the replicas (allowing setting up many addresses will simplify the life users, but it's better to have a single address and a proper load balancer), in case there's no replica, we just forward everything to the primary database.

From a software perspective it's pretty simple, if the operation we're doing is just going to retrieve records, we should use the read-only database, whenever we need to do updates (insert/delete), we must use the primary one.

It would be nice to update everything at once, but there's nothing blocking us from slowly modifying the application to make use of the read-only database.