Add debug logs for VDF duration

src/consensus/pob/vdf.rs

+use std::time::{Duration, Instant};
+
 use ethereum_types::H256;
 use ethnum::U256;
+use tracing::log::debug;
 use vdf::{InvalidIterations, InvalidProof, VDFParams, WesolowskiVDFParams, VDF};
 
 use crate::h256_to_u256;
@@ -38,7 +41,19 @@ pub async fn vdf<'a>(
         if difficulty_check.is_err() {
             return Err(difficulty_check.err().unwrap());
         }
-        wesolowski_vdf.solve(&*challenge, vdf_difficulty)
+        debug!(
+            "Solving wesolowski_vdf, estimated time: {:?} (={:?}min)",
+            Duration::from_secs(vdf_difficulty / VDF_DIFFICULTY_PER_SEC),
+            vdf_difficulty / VDF_DIFFICULTY_PER_SEC / 60
+        );
+        let start = Instant::now();
+        let result = wesolowski_vdf.solve(&*challenge, vdf_difficulty);
+        debug!(
+            "Solved wesolowski_vdf in {:?} (estimated: {:?})",
+            start.elapsed(),
+            Duration::from_secs(vdf_difficulty / VDF_DIFFICULTY_PER_SEC)
+        );
+        result
     })
     .await
     .unwrap()