Pass SendCoinsRecipient (208 bytes) by const reference

Summary:
Completes T559

Backport of PR10964
https://github.com/bitcoin/bitcoin/pull/10964

Test Plan:
make check
test_runner.py

Reviewers: jasonbcox, Fabien, deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2701