When removing the monitor section, or any route that has sends
to it, Session::remove_routes will remove those sends. Despite
Route::remove_processor taking the process lock and removing
the send safely. The send itself will only be destroyed later.
Furthermore the send first calls ~BufferSet() on its
BufferSet mixbufs, before it itself is destroyed.
It was possible to still respond to CycleStart signal which
accessed the mixbufs from the RT thread, causing a heap-use-after-free.