DB: remove shared stuff, add singleton

Add Config class

source/db/db.d

@@ -24,17 +24,17 @@ class DB {
   protected DataSource m_ds;
   protected Connection m_conn;
 
-  shared this(DBSettings settings) {
+  this(DBSettings settings) {
     this.m_settings = settings;
   }
 
-  static shared(DB) getDB(DBSettings settings) {
+  static DB getDB(DBSettings settings) {
     switch (settings.connector) {
     case DBConnector.DB_PGSQL:
       import db.pgsql;
 
       debugF!"DB type is PostgreSQL";
-      return new shared PostgresDB(settings);
+      return new PostgresDB(settings);
 
     default:
       throw new Exception("DB not supported");