Clean up ShardManager/Queuer/Runner #4681

Job	Run time
Test (macOS)	3m 58s
Test (nightly)	3m 10s
Test (stable)	3m 10s
Test (beta)	2m 51s
Test (Windows)	4m 7s
Test (native-tls)	2m 51s
Test (no default features)	45s
Build docs	34s
Test (no cache)	2m 48s
Test (no gateway)	1m 53s
Test (unstable Discord API features)	3m 8s
Examples	3m 28s
Test (simd-json)	3m 8s
Test (chrono, chrono)	45s
Check minimal versions	30s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 6s
Test (builder without model, builder, true)	25s
Test (unstable Discord API (no default features), unstable_discord_api, true)	29s
MSRV	0s
	39m 6s

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