Encapsulate Message Cache in it's own type #5919

Job	Run time
Test (macOS)	56s
Test (nightly)	1m 0s
Test (stable)	1m 17s
Build docs	19s
Test (beta)	50s
Examples	2m 29s
Test (Windows)	3m 27s
Test (native-tls)	1m 13s
Check minimal versions	26s
MSRV	0s
Test (no default features)	43s
Test (no cache)	1m 53s
Test (no gateway)	1m 25s
Test (unstable Discord API features)	1m 11s
Test (zlib compression)	1m 14s
Test (zstd compression)	1m 13s
Test (zlib and zstd compression)	1m 10s
Test (chrono, chrono)	44s
Test (unstable API + typesize, default unstable typesize, true)	50s
Test (builder without model, builder, true)	25s
Test (unstable Discord API (no default features), unstable, true)	25s
	23m 10s

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