[Bug] ConcurrentModificationException with EventWaiter

[Shengaero]

Issue

Issue Checklist

Please follow the following steps before opening this issue.

Issues that do not complete the checklist may be closed without any help.

• I have checked for similar issues on the issue tracker.
• I have updated to the latest version of JDA-Utilities.
• I have checked the branches or the maintainers' PRs for upcoming features/bug fixes.

The issue tracker is reserved for issues, errors, and feature requests related
to JDA-Utilities, and not questions and other requests for help.

• For examples, check out the examples module or the list
  of JDA-Utilities projects.
• For questions join the official JDA discord server and ask them
  in our #jda-utilities channel.
• For general programming questions, visit StackOverflow.

Issue Information

Check all that apply:

• This is a bug report about an error, issue, or bug in JDA-Utilities.
  • I have been able to consistently reproduce this bug.
• This is a feature request for the JDA-Utilities library.

This issue tracker does not assist or handle issues with the JDA library.

For JDA related issues, visit the JDA issue tracker
and open an issue there.

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)