Reintroduce the concept of virtual size from core

Summary: Bitcoin Core use the concept of virtual size to compute custom fees. We do not need to implement a ton of custom logic for our fee if we reuse that work.

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3180