Pass WalletModel down to SendCoinsEntry by construct

Summary: This ensures the wallet model is passed down in constructors so that the SendCoinsEntry has it right away.

Test Plan:

make check

Make sure the ubsan build is now fixed.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3720