[Enhancement] Execute commands in a new thread

[pindab0ter]

Issue

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Description

Currently every command is executed sequentially. In practice this means that sometimes a user will have to wait for a command to finish running which isn't clear to that user. While running heavy code asynchronously within the execute function is a solution for this, a much better solution would be to simply run every command asynchronously.

[jagrosh]

There's a few reasons why this shouldn't be changed:

• For most commands, you don't want to run them asynchronously. Generally you should only run commands async if they do processing that takes a long time or are making external calls; doing everything async can lead to race conditions, and can also have worse performance.
• You can already do this; we provide CommandEvent#async(Runnable) so that you can run the contents of a specific command asynchronously
• If you want to do everything async, you should just write a custom event manager, as JDA-Utils' command client is just an event listener, not an event manager. We shouldn't have the internals of this event listener deviate from the developer's intended event manager system.