store: Add missing buffer flushes.

This could result in deadlock in unusual situations, whereby we'd start
waiting for a reply while the query hasn't been flushed to the socket.

* guix/store.scm (buffering-output-port)[flush]: Add call to
'force-output'.
(add-to-store): Add call to 'write-buffered-output'.

guix/store.scm

@@ -770,6 +770,7 @@ bytevector) as its internal buffer, and a thunk to flush this output port."
 
   (define (flush)
     (put-bytevector port buffer 0 total)
+    (force-output port)
     (set! total 0))
 
   (define (write bv offset count)
@@ -927,6 +928,7 @@ path."
              (write-int (if recursive? 1 0) port)
              (write-string hash-algo port)
              (write-file file-name port #:select? select?)
+             (write-buffered-output server)
              (let loop ((done? (process-stderr server)))
                (or done? (loop (process-stderr server))))
              (read-store-path port)))))