miner: Remove use of CPFP in CreateNewBlock

Summary:

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.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13116