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/authserver/Main.cpp

@@ -76,7 +76,6 @@ boost::asio::deadline_timer* _dbPingTimer;
 uint32 _dbPingInterval;
 boost::asio::deadline_timer* _banExpiryCheckTimer;
 uint32 _banExpiryCheckInterval;
-LoginDatabaseWorkerPool LoginDatabase;
 
 int main(int argc, char** argv)
 {