postgres: Atomically create migrations table

This commit makes `ensureVersionTable` atomic.

Fixes golang-migrate#55

database/postgres/postgres.go

@@ -333,18 +333,7 @@ func (p *Postgres) Drop() error {
 }
 
 func (p *Postgres) ensureVersionTable() error {
-	// check if migration table exists
-	var count int
-	query := `SELECT COUNT(1) FROM information_schema.tables WHERE table_name = $1 AND table_schema = (SELECT current_schema()) LIMIT 1`
-	if err := p.conn.QueryRowContext(context.Background(), query, p.config.MigrationsTable).Scan(&count); err != nil {
-		return &database.Error{OrigErr: err, Query: []byte(query)}
-	}
-	if count == 1 {
-		return nil
-	}
-
-	// if not, create the empty migration table
-	query = `CREATE TABLE "` + p.config.MigrationsTable + `" (version bigint not null primary key, dirty boolean not null)`
+	query := `CREATE TABLE IF NOT EXISTS"` + p.config.MigrationsTable + `" (version bigint not null primary key, dirty boolean not null)`
 	if _, err := p.conn.ExecContext(context.Background(), query); err != nil {
 		return &database.Error{OrigErr: err, Query: []byte(query)}
 	}