Multiple connection support

It would be nice to have support to provide multiple connections with role write or read-only so that it would possible to use for example with PostgreSQL cluster that has single master and multiple read only nodes. Rel would then send write operations to master and select operations to one of slaves

The issue does not name any files, tests, or entry points. Start by locating the connection and query-routing APIs in this Go ORM, then determine how write and read-only roles should be configured and selected for PostgreSQL clusters. Done means documented multi-connection behavior with coverage for routing writes to the master and reads to replicas.

Written by the indexing model from the issue text.