Config different max connc

## Your Question
Is it possible to configure a different MaxConnectionCount for the sources and replica DBs used by the resolver?

## The document you expected this should be explained
I looked at the offical documentation, but it looks like it sets a global MaxConnection for all connections
```
DB.Use(
dbresolver.Register(dbresolver.Config{ /* xxx */ }).
SetConnMaxIdleTime(time.Hour).
SetConnMaxLifetime(24 * time.Hour).
SetMaxIdleConns(100).
SetMaxOpenConns(200)
)
```