Implement `max_concurrency` support when starting shards (#2661) #832

# to view logs

Job	Run time
Test (macOS)	4m 38s
Test (nightly)	3m 5s
Build docs	33s
Test (stable)	3m 9s
Examples	3m 2s
Test (beta)	2m 49s
Check minimal versions	49s
Test (Windows)	5m 47s
MSRV	0s
Test (native-tls)	2m 52s
Test (no default features)	2m 2s
Test (no cache)	2m 50s
Test (no gateway)	1m 49s
Test (unstable Discord API features)	3m 1s
Test (simd-json)	3m 7s
Test (chrono, chrono)	48s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 8s
Test (builder without model, builder, true)	32s
Test (unstable Discord API (no default features), unstable_discord_api, true)	34s
	42m 35s

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