8f2f1e0 wallet: Avoid second mapWallet lookup (João Barbosa)
Pull request description:
All calls to `mapWallet.count()` have the intent to detect if a `txid` exists and most are followed by a second lookup to retrieve the `CWalletTx`. This PR replaces all `mapWallet.count()` calls with `mapWallet.find()` to avoid the second lookup.
Tree-SHA512: 96b7de7f5520ebf789a1aec1949a4e9c74e13683869cee012f717e5be8e51097d068e2347a36e89097c9a89f1ed1a1529db71760dac9b572e36a3e9ac1155f29
Backport of Core PR 11039
https://github.com/bitcoin/bitcoin/pull/11039/files
Completes T550
Depends on D2689