Improve shielded sync (backport #4785)

[mergify]

Describe your changes

• Build commitment tree and witness map locally
  • Uses leanbridgetree crate for this purpose
  • If the number of blocks to sync exceeds a certain threshold, we race with the local approach, by fetching the commitment tree, witness map and notes index in the background
    • Whatever finishes first is the data we store in the shielded wallet (which should have been identical, regardless of being built locally or fetched from the MASP indexer)
• Fix block index not being properly used
  • This is the major source of the speed-up in this PR
• Optimize the layout of the shielded wallet
  • Removed unnecessary fields
• Only fetch block index after a threshold of $N$ blocks (where it makes sense latency wise to wait for that network query vs. fetching $N$ potentially empty blocks)

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

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This is an automatic backport of pull request #4785 done by [Mergify](https://mergify.com).

[mergify]

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