Exercise 2 - 67513 Introduction to Cryptocurrencies

In this project, we have implemented nodes that operate according to the longest-chain protocol.
The nodes function both as wallets and miners. When a node creates a transaction, it is immediately
placed in its mempool and propagated to its neighbors.

The main function of the class is to handle mined blocks from connected nodes.
The method responsible for this is called "notify_of_block" and it's implementation is described in detail below:

    - Step 1: We receive a blockHash from a node and apply the longest chain protocol.
    To do this, we backtrack the node's chain/branch starting from the provided blockhash and search for the
    intersect block of both nodes. We also count how many blocks appear on our branch from get_last_block_hash()
    to the intersection. If the sender's branch is longer than ours, we move to step 2.

    - Step 2: We check that the provided branch of blocks is valid. To do this, we deep copy the utxo of our node
    and update it up to the intersect block. We save a mapping of txid's and object Transactions and each time we see
    a transaction, we replace it with the mapping of its input transaction. By doing this, our time complexity will
    be O(number of blocks from the intersect block to the last_block), which makes more sense than updating
    the utxo from the beginning. We update the node's branch containing the first n valid blocks and recheck if the
    node's branch is still longer than ours. If it is, we move to step 3. We also update our utxo copy with the new
    seen block transactions while checking block validity.

    - Step 3: Now that we know that the node's branch is longer than ours, we need to update our blockchain,
    mempool, and utxo. Our blockchain will be the initial blockchain we were holding up to the intersection block,
    plus the node's branch. Our utxo will be replaced with the copied utxo that we have updated.
    For the mempool, we iterate through our old mempool and remove the transactions that conflict with the new
    updated blockchain. Finally, we move to step 4.

    - Step 4: We inform all our connected nodes about this new blockhash by calling the same method,
     so that all connected nodes get synchronized with the blockchain and utxo.