Remove method SetPool() in favour of sql package's builtin method, Use

it like this `db.DB().SetMaxIdleConns(10)`

So gorm don't need to care about compatibility problems, and you could
use latest `database/sql` features from latest releases without waiting gorm's support

gorm_test.go

@@ -91,7 +91,7 @@ func init() {
 		// CREATE DATABASE 'gorm';
 		// GRANT ALL ON gorm.* TO 'gorm'@'localhost';
 		fmt.Println("testing mysql...")
-		db, err = Open("mysql", "gorm:gorm@/gorim?charset=utf8&parseTime=True")
+		db, err = Open("mysql", "gorm:gorm@/gorm?charset=utf8&parseTime=True")
 	case "sqlite":
 		fmt.Println("testing sqlite3...")
 		db, err = Open("sqlite3", "/tmp/gorm.db")

main.go

@@ -31,12 +31,6 @@ func (s *DB) DB() *sql.DB {
 	return s.db.(*sql.DB)
 }
 
-func (s *DB) SetPool(n int) {
-	if db, ok := s.parent.db.(sqlDb); ok {
-		db.SetMaxIdleConns(n)
-	}
-}
-
 func (s *DB) SetTagIdentifier(str string) {
 	s.parent.tagIdentifier = str
 }

sql.go

@@ -11,7 +11,6 @@ type sqlCommon interface {
 
 type sqlDb interface {
 	Begin() (*sql.Tx, error)
-	SetMaxIdleConns(n int)
 }
 
 type sqlTx interface {