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)