Add default postgres schema so that it can be changed

[lunny]

No description provided.

[lunny]

will be a basis to resolve go-gitea/gitea#2755

[appleboy]

examples:

func init() {
	xorm.DefaultPostgresSchema = "mcs"
}

// NewEngine initializes a new xorm.Engine
func NewEngine() (err error) {
	if err = SetEngine(); err != nil {
		return err
	}

	if err = x.Ping(); err != nil {
		return err
	}

	row, err := x.Exec(`SELECT schema_name FROM information_schema.schemata WHERE schema_name = 'mcs';`)
	if err != nil {
		return err
	}

	affects, err := row.RowsAffected()
	if affects == 0 {
		if _, err = x.Exec(`CREATE SCHEMA "mcs"`); err != nil {
			return err
		}
	}

	if _, err = x.Exec(`SET SEARCH_PATH = mcs, pg_catalog`); err != nil {
		return err
	}

	m := migrate.New(x, migrate.DefaultOptions, migrations)
	if err = m.Migrate(); err != nil {
		return fmt.Errorf("migrate: %v", err)
	}

	if err = x.StoreEngine("InnoDB").Sync2(tables...); err != nil {
		return fmt.Errorf("sync database struct error: %v", err)
	}

	return nil
}

[codecov-io]

Codecov Report

Merging #824 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #824      +/-   ##
==========================================
+ Coverage    53.2%   53.22%   +0.02%     
==========================================
  Files          39       39              
  Lines        7323     7324       +1     
==========================================
+ Hits         3896     3898       +2     
  Misses       2931     2931              
+ Partials      496      495       -1

Impacted Files	Coverage Δ
dialect_postgres.go	77.85% <100%> (+0.08%)	⬆️
xorm.go	68.33% <0%> (+1.66%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5275c0d...1eb2b49. Read the comment docs.