In DispatchSockets(), when there was not enough data, but enough for
current playback, the method would disable the "active" flag so the
next Play() call would re-enable the MultiSocketMonitor.

This was an abuse of the flag which could result in a crash
in Cancel(), because that method asserts that the period_buffer is
empty, which it may be not.

The solution is to add anther flag called "waiting" which shares some
behavior with the old flag.