Readd transactions in the mempool in the right order during reorg.

Summary:
This ensures that no transaction is dropped from the mempool due to its parent not being included yet.

Include various other smaller changes such as using GetId instead of GetHash where apropriate, as well as formatting mempool_packages.py .

Test Plan:

make check
./test/functional/test_runner.py mempool_packages.py

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D989