Allow non-blocking reading of digests (with timeout=0)

[Trigary]

This change lets us call SimpleSwitchP4RuntimeAPI.get_digest_list with very low timeout values, e.g. timeout=0. Currently, a low timeout results in no digest being read (because the timeout is instantly over). With this fix, the timeout is not taken into account if no waiting is necessary because a digest is already available (because a digest has already been received previously).

Motivation: I want to be able to read a digest from one of the switches in the network. If the digest reading is a blocking operation, then it takes a considerable amount of time to iterate over all switches. This simple patch allows users to use digest reading as a non blocking operation, without having to use sockets or select.select directly.