Limit configuration database connections (#16512)

When configuration is backed by a database, a separate pool of
connections is used for access from the primary database. This
change sets sensible config DB connection limits in order to
reduce database resource usage at scale.

config/database.go

@@ -47,6 +47,9 @@ func NewDatabaseStore(dsn string) (ds *DatabaseStore, err error) {
 	if err != nil {
 		return nil, errors.Wrapf(err, "failed to connect to %s database", driverName)
 	}
+	// Set conservative connection configuration for configuration database.
+	db.SetMaxIdleConns(0)
+	db.SetMaxOpenConns(2)
 
 	defer func() {
 		if err != nil {