Core: Backport 6.x realm changes

Make acessible all the info about current realm (e.g name) anywhere, not only realm id
Reduce the number of differences between the two branches

Original changes by Shauren

Partial port of bacc90b6ba and 63def8aa32

src/server/authserver/Main.cpp

@@ -134,7 +134,7 @@ int main(int argc, char** argv)
     // Get the list of realms for the server
     sRealmList->Initialize(*_ioService, sConfigMgr->GetIntDefault("RealmsStateUpdateDelay", 20));
 
-    if (sRealmList->size() == 0)
+    if (sRealmList->GetRealms().empty())
     {
         TC_LOG_ERROR("server.authserver", "No valid realms specified.");
         StopDB();