[wallet] Verify unconfirmed transactions after syncing

Verify the unconfirmed transactions we download against the consensus
rules. This is currently exposed as an extra `verify` feature, since it
depends on a pre-release version of `bitcoinconsensus`.

Closes #352

src/wallet/export.rs

@@ -230,6 +230,7 @@ mod test {
                 timestamp: 12345678,
                 height: 5000,
             }),
+            verified: true,
         })
         .unwrap();