Core/Database: Move singleton instances into compilation units

* Fixes issues when building shared libraries
  (prevents gcc and clang from providing several instance)
* Also instantiate the database worker pools in DatabaseEnv.cpp now.

(cherry picked from commit a8fe7f6eb7)

src/server/database/Database/DatabaseEnv.cpp

@@ -0,0 +1,22 @@
+/*
+ * Copyright (C) 2008-2016 TrinityCore <http://www.trinitycore.org/>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "DatabaseEnv.h"
+
+WorldDatabaseWorkerPool WorldDatabase;
+CharacterDatabaseWorkerPool CharacterDatabase;
+LoginDatabaseWorkerPool LoginDatabase;