Use a callback in client:sync() to determine when to stop the loop

Instead of adding more and more possible conditions on which the while-loop
inside client:sync() could stop, support passing a callback function which is
used to determine when to stop the loop.

The old "so N iterations behaviour" can be achieved with something like:

  function iterate_n_times(n)
    return function ()
      n = n - 1
      return n == 0
    end
  end
  client:sync(iterate_n_times(10))

If no function is passed, the client:sync() function loops forever.

matrix/client.lua

@@ -429,12 +429,16 @@ function Client:_sync(options)
    end
 end
 
-function Client:sync(niters)
-   while niters == nil or niters > 0 do
-      self:_sync { timeout = 15000 }
-      if niters then
-         niters = niters - 1
+local function return_false()
+   return false
 end
+
+function Client:sync(stop, timeout)
+   if not stop then
+      stop = return_false
+   end
+   while not stop(self) do
+      self:_sync { timeout = timeout or 15000 }
    end
 end