Drop upgrade-cancel callback registration for a generic "cancelable"

Summary:
Instead of passing a StartShutdown reference all the way up from txdb,
give ShowProgress a "cancelable" boolean, as StartShutdown is pretty
much always what you'll want to use to cancel. Use the same boolean to
allow cancel during initial block verification.

Backport of core PR10770

Completes task T511

Test Plan:

make check

Start bitcoin-qt and cancel at splashscreen by pressing 'q'

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2476