Fix async command building throwing CME in some cases

[roro1506HD]

The issue is from multiple places during that sendAsync process: Anything that loops through any CommandNode's children is subject to the CME.

The previous fix was only working for the root node, and wasn't even fully working given a few lines lower it's being added in an HashMap, which does .hashCode and loops through the root's children again.

This PR reverts the previous fix and fixes the concurrency issue once for all:

• Changed the CommandNode's children map to a synchronized map, still backed by a LinkedHashMap
• Lock everything in Commands#fillUsableCommands inside a synchronized block that uses the children map as lock object, the same lock object the synchronized map uses.

This has been tested with the test plugin published on this branch of my fork

Fixes #11101