fix(walltime): flip block_timestamp/wall_time_ms (#75)

These variables were flipped from their intended meanings.

See:
https://github.com/transmissions11/reth-walltime/blob/15218b6d66c8da339bab541aa71373aaceb66165/bin/reth/src/optimism.rs#L170-L171

crates/walltime/src/lib.rs

@@ -32,8 +32,8 @@ impl OdysseyWallTime {
         tokio::task::spawn(async move {
             while let Some(notification) = st.next().await {
                 let tip = BlockTimeData {
-                    wall_time_ms: notification.tip().timestamp,
-                    block_timestamp: unix_epoch_ms(),
+                    wall_time_ms: unix_epoch_ms(),
+                    block_timestamp: notification.tip().timestamp,
                 };
                 *listener.inner.block_time_data.write().await = Some(tip);
             }