[wallet] Basic keypool topup

Summary:
backport core's PR11022

Depends on D1047 and D1048

Test Plan:

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

Reviewers: #bitcoin_abc, matiu

Reviewed By: #bitcoin_abc, matiu

Subscribers: matiu, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1049