[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/blockchain/rpc.rs

@@ -233,6 +233,7 @@ impl Blockchain for RpcBlockchain {
                     received,
                     sent,
                     fee: tx_result.fee.map(|f| f.as_sat().abs() as u64),
+                    verified: true,
                 };
                 debug!(
                     "saving tx: {} tx_result.fee:{:?} td.fees:{:?}",

src/blockchain/utils.rs

@@ -362,6 +362,7 @@ fn save_transaction_details_and_utxos<D: BatchDatabase>(
         sent: outgoing,
         confirmation_time: ConfirmationTime::new(height, timestamp),
         fee: Some(inputs_sum.saturating_sub(outputs_sum)), /* if the tx is a coinbase, fees would be negative */
+        verified: height.is_some(),
     };
     updates.set_tx(&tx_details)?;