feat: resolve *sql.DB based on the database context

[lzakharov]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

This PR adds ability to resolve *sql.DB connection based on the database context. It requires go-gorm/gorm#6366 (already in master).

User Case Description

At this moment all these statements return the same connection pool, ignoring provided clauses:

db.DB()
db.Clauses(dbresolver.Write).DB()
db.Clauses(dbresolver.Read).DB()
db.Clauses(dbresolver.Use("slow")).DB()
// etc

However, it should return different connections based on the current database state. See more cases inside TestConnPool.

[lzakharov]

@jinzhu @a631807682 can you review please?

[a631807682]

Can the functions in pool.go be made private? We don't seem to need to export for users to use.

[lzakharov]

Is it possible to merge this PR now? And it would be great to make a release after that.

[lzakharov]

@jinzhu @a631807682 sorry for disturbing again, any updates here?