cache open: handle EAGAIN
... probably via random exponential backoff or something. Details: https://gitter.im/CZ-NIC/knot-resolver?at=5b73e162a37112689c21348b
... probably via random exponential backoff or something. Details: https://gitter.im/CZ-NIC/knot-resolver?at=5b73e162a37112689c21348b