[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/types.rs

@@ -165,6 +165,15 @@ pub struct TransactionDetails {
     /// If the transaction is confirmed, contains height and timestamp of the block containing the
     /// transaction, unconfirmed transaction contains `None`.
     pub confirmation_time: Option<ConfirmationTime>,
+    /// Whether the tx has been verified against the consensus rules
+    ///
+    /// Confirmed txs are considered "verified" by default, while unconfirmed txs are checked to
+    /// ensure an unstrusted [`Blockchain`](crate::blockchain::Blockchain) backend can't trick the
+    /// wallet into using an invalid tx as an RBF template.
+    ///
+    /// The check is only perfomed when the `verify` feature is enabled.
+    #[serde(default = "bool::default")] // default to `false` if not specified
+    pub verified: bool,
 }
 
 /// Block height and timestamp of the block containing the confirmed transaction