Implement the Join Operator

The join operator is also a stateful operator. It keeps the input deltas
stored in the state, for both the left and right branches of the join.

JOINs extract a join key, which is the values that were used in the
join's equality statement. That key is now our zset_id, and it points
to a collection of rows.

core/incremental/dbsp.rs

@@ -75,6 +75,10 @@ impl HashableRow {
 
         hasher.finish()
     }
+
+    pub fn cached_hash(&self) -> u64 {
+        self.cached_hash
+    }
 }
 
 impl Hash for HashableRow {
@@ -168,7 +172,7 @@ impl Delta {
 }
 
 /// A pair of deltas for operators that process two inputs
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Default)]
 pub struct DeltaPair {
     pub left: Delta,
     pub right: Delta,