Add config for the shard start wait time (#3031) #995

Job	Run time
Build docs	26s
Check minimal versions	31s
Test (macOS)	1m 28s
Examples	2m 28s
MSRV	0s
Test (nightly)	1m 26s
Test (stable)	2m 33s
Test (beta)	2m 19s
Test (Windows)	4m 33s
Test (native-tls)	2m 16s
Test (no default features)	51s
Test (no cache)	2m 16s
Test (no gateway)	1m 35s
Test (unstable Discord API features)	2m 30s
Test (chrono, chrono)	45s
Test (unstable API + typesize, default unstable typesize, true)	47s
Test (builder without model, builder, true)	39s
Test (unstable Discord API (no default features), unstable, true)	32s
	27m 55s

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