Add `close-connection'.

* guix/store.scm (close-connection): New procedure.

guix/store.scm

@@ -41,6 +41,7 @@
             hash-algo
 
             open-connection
+            close-connection
             set-build-options
             valid-path?
             add-text-to-store
@@ -280,6 +281,10 @@
                       (process-stderr s)
                       s))))))))
 
+(define (close-connection server)
+  "Close the connection to SERVER."
+  (close (nix-server-socket server)))
+
 (define current-build-output-port
   ;; The port where build output is sent.
   (make-parameter (current-error-port)))