Prevent overfilling of self-pipe in python event loop

Prevent overfilling of self-pipe in python event loop

If the event loop takes a very long time todo something, it is
possible for the 'self pipe' buffer to become full at which
point the entire event loop + remote driver deadlock. Use a
boolean flag to ensure we have strict one-in, one-out behaviour
on writes/reads of the 'self pipe'