If we read no packets, try again after 10ms.

This might help reduce flakiness under heavy load. It
increases the runtime of multinetwork_test from ~16s to ~26s, but
if it fixes the flakiness issues, then we can look at reducing
the 10ms sleep to something shorter.

Change-Id: Ie2b5a28a286ade0979e9b3f0086924495a1058ce