Calculate and store the number of bytes required to spend an input

Summary: This is extracted from Core PR10637 : https://github.com/bitcoin/bitcoin/pull/10637/commits/12ec29d3bb0d46c61712210fe9bb96a0d543204a

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3243