More block template creation simplifications

Summary: This is a small refactor that makes the code easier to reason about by limiting the use of iterators. This causes less dereferences (the code is easier to read) and allow for a mapTx lookup removal.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13401