Use CScheduler for wallet flushing, remove ThreadFlushWalletDB

Summary:
Backport core's PR9605

Depends on D910

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D928