Add _PipeCondition class

_PipeCondition is a condition implemented using pipe(2) and poll(2).
It allows the implementation of timeouts without using a busy-wait loop
with time.sleep.

Unlike Python's built-in threading.Condition class and to save file
descriptors and an internal queue, it can only be used to notify
all waiters. Ganeti's use case for this condition class doesn't
require the ability to notify only one waiter.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>