[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/database/mod.rs

@@ -321,6 +321,7 @@ pub mod test {
                 timestamp: 123456,
                 height: 1000,
             }),
+            verified: true,
         };
 
         tree.set_tx(&tx_details).unwrap();