zcash_client_backend: Implement async wallet synchronization function #5182

Job	Run time
Test on Linux	9m 18s
Test Orchard on Linux	10m 34s
Bitrot check	2m 5s
Build target wasm32-wasi	1m 52s
Test NU6 on Linux	9m 7s
Latest build on ubuntu-latest	1m 57s
Test on macOS	20m 2s
Latest build on windows-latest	3m 47s
Check ZFuture build on Linux	34s
Test Orchard on macOS	24m 52s
Clippy (MSRV)	2m 12s
Latest build on macOS-latest	3m 8s
Test on Windows	13m 55s
Check ZFuture build on macOS	1m 21s
Check ZFuture build on Windows	2m 17s
Test Orchard on Windows	17m 23s
Clippy (beta)	14s
Test NU6 on Windows	14m 48s
Intra-doc links	2m 37s
Code coverage	18m 5s
Rustfmt	16s
protobuf consistency	1m 59s
UUID validity	5s
	2h 42m 28s

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