Allow to cancel the txdb upgrade via splashscreen keypress 'q'

Summary: Backport core's PR10660

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, matiu

Reviewed By: #bitcoin_abc, matiu

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1015