Batch flushing operations to the walletdb during top up and increase keypool size.

Summary: This backport core's PR10831

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/D1045