unrelated technical debt: make sure to run par_iter within the contex…

…t of an existing thread pool (avoid creating a global thread pool if possible)

consensus/src/consensus/mod.rs

@@ -767,12 +767,13 @@ impl ConsensusApi for Consensus {
         let mut pruning_utxoset_write = self.pruning_utxoset_stores.write();
         pruning_utxoset_write.utxo_set.write_many(utxoset_chunk).unwrap();
 
-        // Parallelize processing
-        let inner_multiset =
+        // Parallelize processing using the context of an existing thread pool.
+        let inner_multiset = self.virtual_processor.install(|| {
             utxoset_chunk.par_iter().map(|(outpoint, entry)| MuHash::from_utxo(outpoint, entry)).reduce(MuHash::new, |mut a, b| {
                 a.combine(&b);
                 a
-            });
+            })
+        });
 
         current_multiset.combine(&inner_multiset);
     }

consensus/src/pipeline/virtual_processor/processor.rs

@@ -1210,6 +1210,15 @@ impl VirtualStateProcessor {
             true
         }
     }
+
+    /// Executes `op` within the thread pool associated with this processor.
+    pub fn install<OP, R>(&self, op: OP) -> R
+    where
+        OP: FnOnce() -> R + Send,
+        R: Send,
+    {
+        self.thread_pool.install(op)
+    }
 }
 
 enum MergesetIncreaseResult {