Add temporary cache for messages (#2621) #854

Job	Run time
Examples	3m 37s
Build docs	44s
Test (macOS)	3m 20s
Check minimal versions	31s
MSRV	0s
Test (nightly)	2m 59s
Test (stable)	3m 18s
Test (beta)	3m 14s
Test (Windows)	5m 20s
Test (native-tls)	2m 55s
Test (no default features)	41s
Test (no cache)	3m 15s
Test (no gateway)	1m 49s
Test (unstable Discord API features)	3m 7s
Test (simd-json)	3m 22s
Test (chrono, chrono)	47s
Test (unstable API + typesize, default unstable_discord_api typesize, true)	1m 4s
Test (builder without model, builder, true)	32s
Test (unstable Discord API (no default features), unstable_discord_api, true)	25s
	41m 0s

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