Add support for handling ping responses on long op's

[stromnet]

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.

[miccoli]

Thank stromnet for the fix. I was able to observe this behaviour (OwnetProxy.read sometimes returning an empty string from an owserver under stress) but not to track it down to the described ping frames.

[stromnet]

You're welcome, thanks for merging :)
Btw, any plans for supporting persistent connections?

[miccoli]

Persistent connections are on my todo list, to be implemented via a context manager.

[stromnet]

Great, then I won't look further into doing that right now. I'm interested in the feature, but it's not a blocker at the moment.

Thanks for the library btw, works nice for me since I need to do alarm scans, which I haven't been able to get the native python library to do. And also, the native library is...native, depending on the .so and all.. Nice to not have that dependency!