Merge #10126: Compensate for memory peak at flush time

Summary:
7228ce8 Compensate for memory peak at flush time (Pieter Wuille)

Tree-SHA512: 97e9848410fab061402c85d8440c54a50dd8a0203b2ea194013ea116700a6dc1b4b26b8c5f9c9c68c1f5c6b935c5d6c737437c1911b003d9ff5445c570cd449d

Backport of Core PR10126
https://github.com/bitcoin/bitcoin/pull/10126/files

Test Plan:
make check
test_runner.py

Reviewers: deadalnix, Fabien, #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D2999