change(zebra-scan): Scan only one key per backend call

[oxarbitrage]

Motivation

We want to scan only one key and one block per call to the zcash_client_backend::scanning::scan_block function.

The call support scanning a list of keys which it should be more efficient but we can't relate transaction with key in the results.

Close #8018

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Solution

Replace the list argument of the zebra zcan_block function with single key element.

The scanning_from_populated_zebra_state test case was removed because after the change no empty key will be allowed and the account/nullifier method to scan is not supported; by that the test ends up being the same as scanning_zecpages_from_populated_zebra_state

Testing

Change covered by existing scanning_zecpages_from_populated_zebra_state test, after minor modifications to the call argument.

Review

Anyone can review.

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Follow Up Work

[teor2345]

Thanks!

[teor2345]

The lightwalletd test failed with:

{"app":"lightwalletd","bind_addr":"127.0.0.1:40611","error":"listen tcp 127.0.0.1:40611: bind: address already in use","level":"fatal","msg":"couldn't create listener","time":"2023-11-29T21:21:38Z"}

https://github.com/ZcashFoundation/zebra/actions/runs/7038410028/job/19155707096?pr=8034#step:4:305

This is one of those bugs that happens occasionally when we use the same port as another test or running CI server process.