Make server process killable (#105)

35163c3 introduced a new use of `mask` which makes the server
process uninterruptible while waiting for a new incoming request.
This change fixes that by surrounding the logic that waits for a
new request with `unmask`.  This new `unmask` should still
respect the finalization guarantees of the surrounding masked
code.

core/src/Network/GRPC/LowLevel/Server/Unregistered.hs

@@ -48,7 +48,7 @@ withServerCallAsync :: Server
                     -> (ServerCall -> IO ())
                     -> IO ()
 withServerCallAsync s f = mask $ \unmask ->
-  serverCreateCall s >>= \case
+  unmask (serverCreateCall s) >>= \case
     Left e -> do grpcDebug $ "withServerCallAsync: call error: " ++ show e
                  return ()
     Right c -> do wasForkSuccess <- forkServer s handler