Discourage fee sniping with nLockTime

By default bdk sets the transaction's nLockTime to current_height
to discourage fee sniping.
current_height can be provided by the user through TxParams; if the user
didn't provide it, we use the last sync height, or 0 if we never synced.

Fixes #533

src/wallet/tx_builder.rs

@@ -147,6 +147,7 @@ pub(crate) struct TxParams {
     pub(crate) add_global_xpubs: bool,
     pub(crate) include_output_redeem_witness_script: bool,
     pub(crate) bumping_fee: Option<PreviousFee>,
+    pub(crate) current_height: Option<u32>,
 }
 
 #[derive(Clone, Copy, Debug)]
@@ -543,6 +544,17 @@ impl<'a, D: BatchDatabase, Cs: CoinSelectionAlgorithm<D>, Ctx: TxBuilderContext>
         self.params.rbf = Some(RbfValue::Value(nsequence));
         self
     }
+
+    /// Set the current blockchain height.
+    ///
+    /// This will be used to set the nLockTime for preventing fee sniping. If the current height is
+    /// not provided, the last sync height will be used instead.
+    ///
+    /// **Note**: This will be ignored if you manually specify a nlocktime using [`TxBuilder::nlocktime`].
+    pub fn set_current_height(&mut self, height: u32) -> &mut Self {
+        self.params.current_height = Some(height);
+        self
+    }
 }
 
 impl<'a, D: BatchDatabase, Cs: CoinSelectionAlgorithm<D>> TxBuilder<'a, D, Cs, CreateTx> {