Changes

Add more configurations for JDABuilder (#705)
* Change core pool size setting to rate-limit handling pool
* Added an option to set the okhttp client used by JDA to allow using a
single okhttp client for all JDA sessions
* Add option for setting rate-limit thread-pool through builders
* Ensure the token in DefaultShardManager is only validated once
* Shutdown JDA when token verification fails
* Change build methods in JDABuilder buildAsync -> build() buildBlocking
-> build().awaitReady()
* Use ForkJoinPool for callbacks by default
* Improve getGuildById performance
 (commit: 4cc724e)