Fixes and improvements to world API

[saihv]

Consolidating some bug fixes and improvements to the world API. Work in progress.

• Handle the corner case when UE deletes an actor visually and from the actor list but reference still exists in memory, causing fatal errors when another object of the same name is spawned.
• Queue up garbage collection when an actor is destroyed.
• Use a TMap for name<->asset mapping, instead of iterating through all assets in the database in GetMeshfromRegistry at every spawn.
• Use a TMap for name<->actor mapping, instead of iterating through all actors in scene at every object API call.

[rajat2004]

This broke AirSim on UE 4.24 -

/home/rajat/AirSim/Unreal/Environments/Blocks/Plugins/AirSim/Source/WorldSimApi.cpp:116:40: error: no member named 'NameMode' in 'FActorSpawnParameters'
                new_actor_spawn_params.NameMode = FActorSpawnParameters::ESpawnActorNameMode::Required_ReturnNull;
                ~~~~~~~~~~~~~~~~~~~~~~ ^
/home/rajat/AirSim/Unreal/Environments/Blocks/Plugins/AirSim/Source/WorldSimApi.cpp:116:74: error: no member named 'ESpawnActorNameMode' in 'FActorSpawnParameters'
                new_actor_spawn_params.NameMode = FActorSpawnParameters::ESpawnActorNameMode::Required_ReturnNull;
                                                  ~~~~~~~~~~~~~~~~~~~~~~~^
2 errors generated.
LogInit: Warning: Still incompatible or missing module: Blocks

Are we moving on to 4.25 entirely? In that case, docs need to be updated
Or should this be fixed somehow

[saihv]

@rajat2004 I reverted that change in #3000.