Mempool refactor

[Duddino]

Abstract

This PR aims to better define the role of the Mempool class, and its relationship to other parts of the code. It's worthwhile to note that the name 'Mempool' has been a misnomer for quite a while; I've decided to interpret its role as a Transaction Manager, implementing the following functionalities:

• Storing and managing transactions
• Outpoint management, including the ability to tell which one are unspent, locked and their type (pk2ph/p2cs)
• UTXO retrival
• Calculating and caching balance

The circular dependency with the Wallet class was removed. Now the Wallet class holds a private instance of the Mempool, and all public references to the global mempool object have been removed.

Why not merge this with the Wallet class?

I believe the Wallet class already has a lot of complexity, and it may be more beneficial to abstract some behavior to other classes, rather than having one big class.

Other minor improvements:

• Cache management of the different balances has been simplified, by mapping filter->balance.
• Added unit tests for the new mempool class
• The getUTXOs method was changed. It now takes a filter, which should usually be left as default = OutpointState.SPENT. filter removes any UTXO that matches the OutpointState.
  A new parameter has been added, requirement. This does the opposite of filter, and only returns UTXOs that match the given state. For example getUTXOs({requirement: OutpointState.P2PKH}) would only return regular P2PKH UTXOs

TODO

• Fix masternodes
• Improve mempool unit tests
• Readd tx db
• Change mempool name?

Testing

• Test that sending/receiving transaction works as expected
• Test that txs get locked as expected (Right now the only locked txs are masternode ones)
• Test that balances are correct

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[panleone]

overall the PR is very good, I'm leaving a partial review about algorithms and data structure used.
Most of them can be done as future PRs since the old system did the same and this is a refactor PR

[panleone]

nits/triviliaties

[JSKitty]

tACK bfde392

Looking stable in every way I can test: HD and non-HD wallets, Staking and Unstaking, Shielding and Deshielding, synchronising, all appear to be perfectly stable and reliable.

[panleone]

tACK and merging