[wallet] Add change type to CCoinControl

Summary:
This is extracted from Core PR12194 . The rest of the PR is not interesting for us at this stage, but this part will avoid merge conflicts.

https://github.com/bitcoin/bitcoin/pull/12194/commits/31dbd5af4863d73c3215355ec5cf85bde2ddc5d8

Depends on D3661

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3662