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