Don't use pass by reference to const for cheaply-copied types (bool, char, etc.).

Summary:
Backport Core PR10839
https://github.com/bitcoin/bitcoin/pull/10839/files
Completes T508

Test Plan: make check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2427