Concurrency updates

[TPGamesNL]

Description

A few changes to AsyncEffect:

• Update javadoc with a notice to ScriptLoader.hasDelayBefore = Kleenean.TRUE (otherwise the parser will think the event isn't delayed while it is).
• Make local variables available in the effect itself, not just after it.
• Cleaned up the code a bit.

Also updated EffLoadServerIcon to not throw an exception when its argument evaluated to null, and to set the hasDelayBefore field appropriately.

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Target Minecraft Versions: any
Requirements: none
Related Issues: #3808 #1817

[TPGamesNL]

Just noticed

Skript/src/main/java/ch/njol/skript/variables/Variables.java

Lines 244 to 247 in 885a4b6

	/**
	* Not to be accessed outside of Bukkit's main thread!
	*/
	private final static Map<Event, VariablesMap> localVariables = new HashMap<>();

[TPGamesNL]

@FranKusmiruk Should I edit VariablesMap to use ConcurrentHashMap and ConcurrentSkipListMap instead of HashMap and TreeMap?

PS PR didn't build because of downtime at Maven repo used by PaperLib

[FranKusmiruk]

@FranKusmiruk Should I edit VariablesMap to use ConcurrentHashMap and ConcurrentSkipListMap instead of HashMap and TreeMap?

I believe that's out of the scope of this PR and still up for debate. I'd rather leave it for another PR so the bug fixes on this one can be merged faster.