[wallet] Close bdb when flushing wallet. (John Newbery)
Pull request description:
bdb would not be closed when closing the wallet in wallet-tool. Fix this by calling wallet->flush with true.
https://github.com/bitcoin/bitcoin/pull/15357/commits/dfbf117bbb82c76c8cb60dba73ae7d653e3b8348
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Backport of Core PR15390