Restore Merkle tree pruning for the wallet

[SupremoUGH]

Reenables the Merkle tree pruning functionality for the wallet, this time as a BTreeMap instead of vec, which allows for almost 100% saved space (as opposed to 50%). Fixes some things in fork which were incompatible with pruning, as well as some indexing issues which were incompatible with the branches.

Should also speed up sync time because of the batch optimization when merging a branch into its trunk. Not now, see the issue: #354

Goes together with: Manta-Network/sdk#133

Note for reviewers

When reviewing, please make sure that the indexing is right, meaning: indexes in the Partial Merkle tree and indexes in its leaf map are off by self.starting_leaf_index(). Make sure each function is using the right index, depending on whether they are interacting with the tree or just with its LeafMap.

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Before we can merge this PR, please make sure that all the following items have been checked off:

• Linked to an issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Added one line describing your change in CHANGELOG.md and added the appropriate changelog label to the PR.
• Re-reviewed Files changed in the GitHub PR explorer.
• Checked that changes and commits conform to the standards outlined in CONTRIBUTING.md.

[Apokalip]

lgmt, I dont know if its not a bad idea to have tests for the leaf indexes(reviewer note)