Parallelize more source paths

When looking for source paths to parallelize, the optimizer.walkEntries
examines only the first operator in a sequence.  This prevents the
optimizer from parallelizing a pool scan in the right input of a join
expressed in subquery syntax, like

    from left | join (from right | ...) on left_key=right_key

Change walkEntries to examine all operators in a sequence so almost all
source paths are parallelized.  (The exceptions are paths inside
dag.Over and paths containing no operator after the source.)

compiler/optimizer/optimizer.go

@@ -92,8 +92,8 @@ func walk(seq dag.Seq, over bool, post func(dag.Seq) dag.Seq) dag.Seq {
 }
 
 func walkEntries(seq dag.Seq, post func(dag.Seq) (dag.Seq, error)) (dag.Seq, error) {
-	if len(seq) > 0 {
-		switch op := seq[0].(type) {
+	for _, op := range seq {
+		switch op := op.(type) {
 		case *dag.Fork:
 			for k := range op.Paths {
 				seq, err := walkEntries(op.Paths[k], post)
@@ -102,7 +102,6 @@ func walkEntries(seq dag.Seq, post func(dag.Seq) (dag.Seq, error)) (dag.Seq, err
 				}
 				op.Paths[k] = seq
 			}
-			return seq, nil
 		case *dag.Scatter:
 			for k := range op.Paths {
 				seq, err := walkEntries(op.Paths[k], post)
@@ -111,7 +110,6 @@ func walkEntries(seq dag.Seq, post func(dag.Seq) (dag.Seq, error)) (dag.Seq, err
 				}
 				op.Paths[k] = seq
 			}
-			return seq, nil
 		case *dag.Scope:
 			seq, err := walkEntries(op.Body, post)
 			if err != nil {
@@ -253,9 +251,6 @@ func (o *Optimizer) optimizeSourcePaths(seq dag.Seq) (dag.Seq, error) {
 		case *kernel.Reader:
 			op.Filter = filter
 			seq = append(dag.Seq{op}, chain...)
-		case *dag.LakeMetaScan, *dag.PoolMetaScan, *dag.HTTPScan:
-		default:
-			return nil, fmt.Errorf("internal error: an entry point to the query is not a source: %T", op)
 		}
 		return seq, nil
 	})

compiler/ztests/par-join.yaml

@@ -0,0 +1,35 @@
+script: |
+  export ZED_LAKE=test
+  zed init -q
+  zed create -q -orderby ts test
+  # At the time of writing, the where operator is necessary because a pool scan
+  # is parallelized only when followed by another operator.
+  zc -C -P 2 "from test | join (from test | where true) on a=b" | sed -e 's/pool .*/.../'
+
+outputs:
+  - name: stdout
+    data: |
+      lister ...
+      | slicer
+      | scatter (
+        =>
+          seqscan ...
+        =>
+          seqscan ...
+      )
+      | merge ts:asc
+      | fork (
+        =>
+          pass
+        =>
+          lister ...
+          | slicer
+          | scatter (
+            =>
+              seqscan ...
+            =>
+              seqscan ...
+          )
+          | merge ts:asc
+      )
+      | join on a=b