Bump version to 0.19.1

Summary: See title

Test Plan:
make check
Check that bitcoind --version returns 0.19.1

Reviewers: deadalnix, Fabien, Mengerian, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2580