Avoid updating merkle paths of spent notes

[sug0]

Describe your changes

Avoid updating merkle paths of spent notes. This should optimize the synchronous path of the shielded sync on the ledger client.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.92%. Comparing base (b2332bf) to head (550e03a).
Report is 44 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4018      +/-   ##
==========================================
- Coverage   73.92%   73.92%   -0.01%     
==========================================
  Files         341      341              
  Lines      106510   106520      +10     
==========================================
+ Hits        78740    78741       +1     
- Misses      27770    27779       +9     

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[sug0]

I think this code might be subtly wrong... The nullifier map (nf_map) only gets updated after calling save_decrypted_shielded_outputs, but save_shielded_spends depends on the nf_mapto add data to the set of spent notes (spents).

The point of reordering the call to save_shielded_spends in this PR was to remove entries from the witness map before new witnesses were appended during a call to update_witness_map. Might be best to remove the outdated witnesses inside of update_witness_map instead.

After some reflection with @grarco, we've concluded that this code is probably doing the right thing. For the time being, I'll leave the PR as a draft though.

[murisi]

This code looks okay to me if there is no special interaction between this optimization and the speculation code (which I've not looked into before). save_shielded_spends being called earlier should be okay since the new notes saved in save_decrypted_shielded_outputs can only be nullified by a Transaction coming after the one being processed in the current loop iteration.