Move some static functions out of wallet.h/cpp
Backport of Core PR10976
This change is desirable because wallet.h/cpp are monolithic and hard to
navigate, so pulling things out and grouping together pieces of related
functionality should improve the organization.
Another proximate motivation is the wallet process separation work in
https://github.com/bitcoin/bitcoin/pull/10973, where (at least initially)
parameter parsing and fee estimation are still done in the main process rather
than the wallet process, and having functions that run in different processes
scrambled up throughout wallet.cpp is unnecessarily confusing.
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, deadalnix, jasonbcox
Subscribers: jasonbcox, deadalnix, teamcity
Differential Revision: https://reviews.bitcoinabc.org/D1785