optim: mark a few classes final

Summary:
Enable devirtualization opportunities by using the final specifier (C++11)

• Declaring CCoinsViewErrorCatcher final enables devirtualization of two calls
• Declaring CReserveKey final enables devirtualization of one call

Core PR10809

Fixes T171

Test Plan: make check && test/functional/test_runner.py

Reviewers: schancel, deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Maniphest Tasks: T171

Differential Revision: https://reviews.bitcoinabc.org/D1016