feat: allow to specify read-only replica for SELECTs

[vmihailenco]

There are some minor API changes. Most notable of them is the removal of GetConn() IConn method, because the internal query conn IConn must be initialized to nil so Bun can resolve to a random read-only connect.

Overwriting the connection using query.Conn(conn) still works.

The usage should be like this:

db := bun.NewDB(sqldb, dialect,
  bun.WithReadOnlyReplica(sqldb2),
  bun.WithReadOnlyReplica(sqldb3))

When/if we want to provide custom logic how to pick a replica (e.g. replica with the lowest latency), it can be done like this:

db := bun.NewDB(sqldb, dialect,
  bun.WithConnResolver(resolver))

Closes #1037

[Tiscs]

Awesome.

I think variadic function is a good way to allow multiple replicas to be set at once.

db := bun.NewDB(sqldb, dialect,
  bun.WithReadOnlyReplicas(sqldb2),
  bun.WithReadOnlyReplicas(sqldb3, sqldb4),
  bun.WithReadOnlyReplicas(replicas...))

[j2gg0s]

When we want to use the master for a specific query to avoid master-slave replication delay, how should we proceed?

[vmihailenco]

When we want to use the master for a specific query to avoid master-slave replication delay, how should we proceed?

There is a Conn() method on each query that allow to specify the connection that will be used. So you can just:

db.NewSelect().Conn(db)