Introduce and track a billable size in addition to real txn size

Summary:
Introduce a billable size which can be used to discount, and or increase, fees on specific
sections of a transaction. The future intent is to reduce overall fees, put increase fees
on UTXO generation.

Depends on D1859, D1892

Test Plan:

make VERBOSE=1 check && ./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1860