viewer: switch to modern fetch API, do not cache metadata and entity … #109

Job	Run time
fmt	11s
docs	18s
clippy	27s
build (macos-11, aarch64-apple-darwin)	1m 13s
viewer	11s
build (macos-11, x86_64-apple-darwin)	1m 3s
test (ubuntu-latest, stable)	25s
build (windows-2019, x86_64-pc-windows-msvc, .exe)	2m 53s
test (windows-latest, stable)	1m 9s
build (windows-2019, i686-pc-windows-msvc, .exe)	1m 37s
test (macOS-latest, stable)	40s
build (ubuntu-20.04, x86_64-unknown-linux-gnu)	29s
	10m 36s

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