Bump version to 0.17.1

Summary: We really shouldn't wait before bumping version, this should happen as soon as a released is done to be ready for the next one.

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1272