Fix explosions delayed on special monster death effects

[leoCottret]

Summary

SUMMARY: Bugfixes "Fix explosions delayed on special monster death effects"

Purpose of change

Fixes #2189
The problem is linked to #2092 , on top of the fact that explosions from monster deaths are delayed by one turn, when they should happen instantly.

A quick explanation:
PR 2092 added the tracking of the source of an explosion (the caster of the spell, the thrower of a grenade, a monster that triggers a special attacks etc.).
This allowed kills from explosives to be added to the player's list.
Before that, explosions were supposed to have no origin/source.
Problem: explosions from monsters death are assumed to happen instantly, not 1 turn after.

Describe the solution

Trigger the explosion just after a monster death, for monsters with on_death effects:

• "EXPLODE" (eg cleanerbot)
• "FOCUSEDBEAM" (eg milspec searchlight)

Testing

• Change radius of explosion to 40 to be sure that the player is caught in the explosion (necessary for the crash to happen)
• Kill a cleanerbot, no crash
• Kill a milspec searchlight, no crash

Additional context

This PR does fix the delay problem, which also solves the crash problem, but a monster using some kind of delayed explosion attack could produce the same crash if it dies before the explosion.

[Coolthulhu]

Seems that we should have explosion queue processing after any explosion is queued, unless the queue is being processed right now. A simple volatile bool in explosion queue class should work.
As in:

• Queue an explosion
• Check if we're processing the queue
• If not, start doing it at the end of "enqueue explosion" function
• If yes, just return

[olanti-p]

Seems that we should have explosion queue processing after any explosion is queued, unless the queue is being processed right now.

That would defeat the entire point of the queue: to delay explosion processing until after previous world update operation has finished, which is what was causing #175 (vehicle was being exploded mid-processing).