Speed up rejoins by not calling ParseEntities

[elexis]

A rejoining client

• loads all entities from the XML file
• or runs the random map script to compute the entities
• creates entities with all their components in the simulation

The last step is done in ParseEntities() and can be significantly timeconsuming.

But after that stage all entities are replaced with the deserialized entities.

So if I'm not entirely misjudging the situation, it is entirely useless to call this function for rejoining clients and create these thousands of entities.

(This ticket may become superseded by #2147, i.e. in case we decide to serialize the terrain and the rest of the data of the mapfiles., Then we can avoid the entire LoadMap / LoadRandomMap call in `MapReader.cpp' for rejoiners.)