Add support for handling ping responses on long op's

When owserver is busy (i.e. high load/contention, or long ops), it will
send ping frames every second or so.

How to reproduce:

1. Run pyownet, with verbose, and repeatedly read a temperature sensor (/uncached/nnnn/temperature).
2. Run separate console which does while true; do owread /uncached/nnnn/temperature; done

(or just two pyownet I guess). You will eventually notice that it receives packages with -1 payload.