```
This removes the use of CPFP during block generation. Instead, transactions
are evaluated for block inclusion on individual bases by modified fee rate.
```
Port of bchn#1078.
The code has been largely massaged, with 2 main differences:
- We use CTxMempoolEntry at some places instead of the iterator,
because this is what our API returns/expects.
- The 1-time lambdas are removed to make the code easier to read (with
a single exception).
There will be more cleaning up in a follow up.