Better prefix parsing and event waiter optimizations!

Welcome back to another release.

This implements the following PRs:
#13 - Better prefix parsing and handling, thanks @freya022
#12 - EventWaiter optimizations in multithreaded environments (thanks @freya022)

This also deprecates SlashCommand#guildId as noted in #8. The functionality works and will work until 1.23.0 where it will be fully deprecated (unusable, but not removed) until 2.0. Please use CommandClientBuilder#forceGuildOnly(server ID) instead.