[wallet] setup wallet background flushing in WalletInit directly
Summary:
WalletInit::Start calls postInitProcess() for each wallet. Previously each call to postInitProcess() would attempt to schedule wallet background flushing. Just start wallet background flushing once from WalletInit::Start().
Partial backport of core PR10740 (commit 470316c)
https://github.com/bitcoin/bitcoin/pull/10740/commits/470316c3bf5ca343d5d66b94839169a4572eceb7
Progress towards T459
Test Plan:
make check ./test/functional/test_runner.py wallet_*
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3630