Implement BlockCache trait for FsBlockDb #5974

Job	Run time
Build target wasm32-wasi	52s
Check ZFuture build on Linux	1m 58s
Check ZFuture build on macOS	1m 43s
Check ZFuture build on Windows	5m 50s
Bitrot check	1m 29s
Latest build on ubuntu-latest	2m 18s
Latest build on windows-latest	5m 13s
Clippy (MSRV)	1m 18s
Latest build on macOS-latest	1m 46s
Clippy (beta)	11s
Code coverage	1h 1m 33s
Intra-doc links	2m 2s
Rustfmt	17s
protobuf consistency	1m 0s
UUID validity	3s
Test on Linux	4m 45s
Test Orchard on Linux	3m 46s
Test NU6 on Linux	3m 47s
Test on macOS	4m 58s
Test Orchard on macOS	5m 0s
Test on Windows	4m 50s
Test Orchard on Windows	2m 18s
Test NU6 on Windows	2m 32s
	1h 59m 29s

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