target_spawn_relay does not properly support mapscripting

[Aciz]

There are two main issues:

• Deleting a target_spawn_relay entity does not invalidate the pointer to the spawn relay entity in the spawnRelayEntities struct, which causes a crash.
• Deleting & creating a new target_spawn_relay entity is not possible due to the entity validation that is performed, because the delete call must be slightly delayed to let the entity actually spawn, which means the newly created entity will conflict with the old one.

The first issue is rather simple to solve, but the second isn't so trivial. Implementing #1750 will resolve the need to perform this type of action however.