Rename worker threads to thread pool

I think this needs more work, maybe the thread pool should be more
similar to the resource pool, but I think the name change is still
helpful.

Maybe there's a need for a variable size thread pool and that can
better integrate with the work queue.

tests/thread-pool.scm

@@ -0,0 +1,44 @@
+(use-modules (tests)
+             (srfi srfi-71)
+             (fibers)
+             (unit-test)
+             (knots thread-pool))
+
+(let ((thread-pool
+       (make-thread-pool 2)))
+
+  (run-fibers-for-tests
+   (lambda ()
+     (assert-equal
+      (call-with-thread
+       thread-pool
+       (lambda ()
+         4))
+      4))))
+
+(let ((thread-pool
+       (make-thread-pool
+        2
+        #:thread-initializer (const '(2)))))
+
+  (run-fibers-for-tests
+   (lambda ()
+     (assert-equal
+      (call-with-thread
+       thread-pool
+       (lambda (num)
+         (* 2 num)))
+      4))))
+
+(let ((process-job
+       count-jobs
+       count-threads
+       list-jobs
+       (create-work-queue
+        2
+        (lambda (i)
+          (* i 2)))))
+
+  (process-job 3))
+
+(display "thread-pool test finished successfully\n")