Add PredicateTxGenerator

[tokatoka]

This PR adds PredicateTxGenerator that

1. probes the transactions inside the mempools
2. based on the mempools's utxos, we decide to chain more transactions to it. How we choose the transactions to put onto is decided by a predicate given by the user of this module

for example:

• PredicateTxGenerator::double_spend() is the same as the previous DoubleSpendGenerator that spends a transaction who has another existing transactions that spend it.
• PredicateTxGenerator::chain_spend() extends the chain of transactions, it looks for a transaction that nobody spends, and if it is found, it just spends it.