zcash_client_backend: Allow sharing instances of ScanningKeys between threads

[upbqdn]

Motivation

It would be handy if Zebra could share instances of ScanningKeys between threads.

Solution

• Bound ScanningKeyOps by Send + Sync so ScanningKeys auto-implements Send and Sync instead of !Send and !Sync.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.13%. Comparing base (976a4d2) to head (99dfbb9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1400   +/-   ##
=======================================
  Coverage   63.13%   63.13%           
=======================================
  Files         126      126           
  Lines       14720    14720           
=======================================
  Hits         9293     9293           
  Misses       5427     5427           

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[str4d]

Is this bound (repeated everywhere) avoidable if we add the equivalent bound on AccountId in ScanningKeyOps?

[upbqdn]

Unfortunately, to be able to move an instance of ScannningKeys to tokio::task::spawn_blocking for example, the compiler requires Send and Sync to be implemented for ScanningKeyOps. Otherwise, I get this error:

`(dyn ScanningKeyOps<sapling_crypto::note_encryption::SaplingDomain, AccountId, sapling_crypto::note::nullifier::Nullifier> + 'static)` cannot be sent between threads safely

I tried bounding AccountId by Send and Sync in ScanningKeyOps, but that didn't help.

Bounding ScanningKeyOps itself eliminates the error, but once I do that, I have to make the rest of the changes in this PR to fix subsequent bound requirements.

[upbqdn]

This PR is not important though, feel free to close it without merging if it's of no use. The workaround was to create an instance of ScanningKeys in the scanning thread itself. Moreover, we have no plans to work on the scanner any time soon.