storage/sql: rework of the original MySQL PR

.travis.yml

Documentation/storage.md

cmd/dex/config_test.go

go.mod

go.sum

storage/sql/config.go

storage/sql/config_test.go

storage/sql/postgres_test.go

storage/sql/sql.go

vendor/github.com/go-sql-driver/mysql/.gitignore

vendor/github.com/go-sql-driver/mysql/.travis.yml

vendor/github.com/go-sql-driver/mysql/AUTHORS

vendor/github.com/go-sql-driver/mysql/CHANGELOG.md

vendor/github.com/go-sql-driver/mysql/CONTRIBUTING.md

vendor/github.com/go-sql-driver/mysql/LICENSE

vendor/github.com/go-sql-driver/mysql/README.md

vendor/github.com/go-sql-driver/mysql/appengine.go

vendor/github.com/go-sql-driver/mysql/auth.go

vendor/github.com/go-sql-driver/mysql/buffer.go

vendor/github.com/go-sql-driver/mysql/collations.go