zcash_client_backend: Implement async wallet synchronization function #5179

Job	Run time
Build target wasm32-wasi	1m 52s
Test on Linux	2m 20s
Test Orchard on Linux	1m 25s
Bitrot check	2m 7s
Latest build on ubuntu-latest	1m 26s
Check ZFuture build on Linux	2m 26s
Latest build on windows-latest	1m 26s
Check ZFuture build on macOS	3m 6s
Latest build on macOS-latest	1m 44s
Clippy (MSRV)	2m 11s
Check ZFuture build on Windows	4m 48s
Test NU6 on Linux	1m 27s
Clippy (beta)	12s
Code coverage	4m 18s
Intra-doc links	2m 36s
Test on macOS	2m 40s
Test Orchard on macOS	2m 40s
Test on Windows	2m 24s
protobuf consistency	1m 57s
Rustfmt	15s
Test Orchard on Windows	16s
Test NU6 on Windows	1s
UUID validity	4s
	43m 41s

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