[cashtab] Improve PayButton user experience

Summary: By broadcasting the to appropriated node so the notifaction comes with no delay.

Test Plan:

./contrib/teamcity/build-configurations.py cashtab-tests

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman, Klakurka

Differential Revision: https://reviews.bitcoinabc.org/D18505