Avoid fiber deadlocks

Channels don't represent some channel on which messages travel, at least not a
very long one because it can't accommodate any messages. They simply represent
a direct exchange of the message between a sender and receiver. Because of
this, put-message blocks the fiber, and if all the threads on the other end
are waiting for replies to be received, then you have a deadlock.

To avoid this situation, spawn new fibers to send the messages. I think this
works at least, although I'm unsure how sensible it is.

guix-data-service/utils.scm

@@ -99,7 +99,9 @@
 (define (defer-to-thread-pool-channel thunk)
   (make-thread-pool-channel!)
   (let ((reply (make-channel)))
-    (put-message %thread-pool-channel (cons reply thunk))
+    (spawn-fiber
+     (lambda ()
+       (put-message %thread-pool-channel (cons reply thunk))))
     reply))
 
 (define (fetch-result-of-defered-thunk reply-channel)