Allow nesting worker thread calls

Incorporating changes from the nar-herder.

tests/worker-threads.scm

@@ -4,8 +4,8 @@
              (unit-test)
              (knots worker-threads))
 
-(let ((worker-thread-channel
-       (make-worker-thread-channel
+(let ((worker-thread-set
+       (make-worker-thread-set
         (const '())
         #:parallelism 2)))
 
@@ -13,7 +13,7 @@
    (lambda ()
      (assert-equal
       (call-with-worker-thread
-       worker-thread-channel
+       worker-thread-set
        (lambda ()
          4))
       4))))