Rename GetLogger() to LogInstance()

Summary:
Using the same name than core will make backports easier.

This is extracted from core PR15266:
https://github.com/bitcoin/bitcoin/pull/15266/files

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4532