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.

Makefile.am

@@ -9,7 +9,7 @@ SOURCES =			\
   knots/resource-pool.scm	\
   knots/timeout.scm		\
   knots/web-server.scm		\
-  knots/worker-threads.scm
+  knots/thread-pool.scm
 
 SCM_TESTS =			\
   tests/non-blocking.scm	\
@@ -20,7 +20,7 @@ SCM_TESTS =			\
   tests/web-server.scm		\
   tests/parallelism.scm	\
   tests/resource-pool.scm	\
-  tests/worker-threads.scm
+  tests/thread-pool.scm
 
 TESTS_GOBJECTS = $(SCM_TESTS:%.scm=%.go)