zcash_client_sqlite: Add methods for fixing broken note commitment trees.

[nuttycom]

This functionality is needed in order to make it possible for wallets that have corrupted note commitment tree state to recover from that situation.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 121 lines in your changes missing coverage. Please review.

Project coverage is 53.96%. Comparing base (35c9af8) to head (b006e39).

Files with missing lines	Patch %	Lines
zcash_client_sqlite/src/wallet/common.rs	0.00%	53 Missing ⚠️
zcash_client_sqlite/src/wallet/commitment_tree.rs	0.00%	25 Missing ⚠️
zcash_client_sqlite/src/wallet.rs	0.00%	21 Missing ⚠️
zcash_client_sqlite/src/lib.rs	0.00%	17 Missing ⚠️
zcash_client_sqlite/src/wallet/sapling.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1709      +/-   ##
==========================================
- Coverage   54.27%   53.96%   -0.31%     
==========================================
  Files         179      179              
  Lines       21253    21374     +121     
==========================================
  Hits        11535    11535              
- Misses       9718     9839     +121     

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[str4d]

This appears to work: the Nil node in my buggy repo gets replaced, and then merged into its parent due to the subtree being ephemeral.

However, the branch it is within does not get fully merged, because the act of re-scanning the previously-scanned range causes a frontier to be inserted, which marks the leaf just before the re-scanned range as Reference and thus not to be pruned. We will never mark that node as anything else because the block prior to the re-scanned range is itself already scanned (and thus we won't be inserting any scanned blocks that will overlap with and replace the Reference marking).

I suspect the solution here is to check the scan status of the block containing the frontier when inserting it, and only marking the frontier as Reference when we need to keep it around. Alternatively we could just not insert the frontier at all in this scenario?