[backport#15390] [wallet-tool] Close bdb when flushing wallet

Summary:
[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.

Backport of Core PR15390

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7327