Tweak the resource pool

Mostly to no longer sleep in the main fiber. Now the main fiber just
spawns other fibers when it would previously block on put-operation
and these other fibers communicate back to the main resource pool
fiber when necessary.

This should mean that the resource pool is more responsive.

tests/resource-pool.scm

@@ -15,6 +15,19 @@
                                res)
       2))))
 
+(run-fibers-for-tests
+ (lambda ()
+   (let ((resource-pool (make-resource-pool
+                         (lambda ()
+                           2)
+                         1
+                         #:add-resources-parallelism 1)))
+     (assert-equal
+      (with-resource-from-pool resource-pool
+                               res
+                               res)
+      2))))
+
 (let* ((error-constructor
         (record-constructor &resource-pool-timeout))
        (err