add(mempool): Verify orphaned mempool transactions

[arya2]

Motivation

We want to verify transactions in the mempool that spend the transparent outputs of other transactions in the mempool.

Closes #8777.

Solution

• Adds a handle to the mempool service in the transaction verifier
• Adds AwaitOutput request in the mempool service
• Updates the spent_utxos() method in the transaction verifier to try querying the mempool for spent outpoints before returning a TransparentInputNotFound error.
• Polls the mempool service from the transaction verifier after verifying a mempool transaction that creates new UTXOs (in case they're spent by other mempool transactions that are still being verified)
• Updates getblocktemplate method and ZIP-317 transaction selection to include transactions that spent the outputs of other transactions in the mempool when all of their dependencies have been selected

Tests

Added tests to check that:

• Inserting a transaction also updates the transaction dependencies tracked in the verified set
• The Storage.remove() method works as expected (the rest of evict_one() should be covered already)
• The mempool responds to AwaitOutput requests when the output for the queried outpoint is:
  • Added to the mempool
  • Already available in the mempool
• The transaction verifier:
  • Calls the mempool with AwaitOutput requests as expected and processes UnspentOutput responses correctly
  • Polls the mempool after verifying a mempool transaction that creates new transparent outputs
  • Includes all of the expected spent_mempool_outpoints in its response
• Long poll ids remaining consistent as long the mempool contents are unchanged (and everything else is unchanged)
• ZIP-317 mempool transaction selection excludes transactions that have dependencies unless their dependencies have already been selected

If needed, this PR could also add tests for:

• ZIP-317 mempool transaction selection always returns the right order of transactions (this could be unnecessary since they’re sorted afterwards anyway but it could be a good sanity check)
• The getblocktemplate method returns transactions with dependencies in the right order when the transaction hashes would have them sorted in the wrong order
• Ideally at least a basic acceptance test that checks that it all works via RPCs

Follow Up Work

The mempool's Downloads sends a result to the response channel too early, it should wait until the transaction is being inserted into the mempool's verified set.

PR Author's Checklist

• The PR name will make sense to users.
• The PR provides a CHANGELOG summary.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.

PR Reviewer's Checklist

• The PR Author's checklist is complete.
• The PR resolves the issue.