zcb::sync: Refresh UTXOs at the start of each scanning cycle

[str4d]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 53 lines in your changes missing coverage. Please review.

Project coverage is 61.07%. Comparing base (e7abed3) to head (7026aeb).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
zcash_client_backend/src/sync.rs	0.00%	53 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1536      +/-   ##
==========================================
- Coverage   61.26%   61.07%   -0.19%     
==========================================
  Files         146      146              
  Lines       16890    16942      +52     
==========================================
  Hits        10347    10347              
- Misses       6543     6595      +52     

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[nuttycom]

utACK with a minor note about how to handle birthday heights (which may not be specifically applicable to this PR.)

[nuttycom]

We should set Sapling activation (or maybe Sapling activation + 1) as the minimum height to use for block retrieval in scanning. IIRC there's currently a bug where lightwalletd won't return the frontiers prior to Sapling activation correctly.

[str4d]

I think that already happens elsewhere, or if not then it should? Block retrieval is entirely dictated by the scan queue, so what matters is how the queue is filled; account creation (and the birthday heights provided therein) is what defines the earliest entries in the scan queue.