[Bug] ConcurrentModificationException with EventWaiter

[Shengaero]

Issue

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Description

I am opening this issue after closing #57 as it will require a proper fix and not a simple 2 line patch.

As was previously stated in that PR, EventWaiter is currently not thread safe.

java.util.ConcurrentModificationException: null
    at java.util.HashMap$HashIterator.nextNode(HashMap.java:1442)
    at java.util.HashMap$KeyIterator.next(HashMap.java:1466)
    at java.util.AbstractCollection.toArray(AbstractCollection.java:196)
    at com.jagrosh.jdautilities.commons.waiter.EventWaiter.onEvent(EventWaiter.java:239)