distsql: support window functions in distsql

Adds support of window functions in DistSQL.
A stage of windower processors is added
for a particular PARTITION BY scheme, all
window functions with the same partitioning
are processed by that stage. We also cache
sorted partitions if more than one window
function has the same ORDER BY clause.

A doc describing the details of the
implementation is out.

Note: memory accounting is not done,
I plan on adding that after this PR.

Resolves: cockroachdb#24425.

Release note: None

pkg/sql/distsqlrun/processors.go

@@ -1052,6 +1052,12 @@ func newProcessor(
 		}
 		return newProjectSetProcessor(flowCtx, processorID, core.ProjectSet, inputs[0], post, outputs[0])
 	}
+	if core.Windower != nil {
+		if err := checkNumInOut(inputs, outputs, 1, 1); err != nil {
+			return nil, err
+		}
+		return newWindower(flowCtx, processorID, core.Windower, inputs[0], post, outputs[0])
+	}
 	return nil, errors.Errorf("unsupported processor core %s", core)
 }