Process transactions in two loops in ConnectBlock instead of one.

Summary:
This is a first step toward getting rid of transaction ordering.

Depends on D1479

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1484