Fix stats/socket callback types

[phile314]

See #165 .

I ran my program with these changes for ~30 minutes and everything worked as it should.

[AlexeyRaga]

Thanks, looks good!
Do you know if we should do the same for all the other callbacks that we have here? Like, log callback, offsets committed callback, rebalance callback etc.?
Or the issue is specific for stats callback?

[phile314]

The issue is specific to the stats/socket callback. I checked the other callbacks and I didn't see any issues there:

-rebalance
-delivery
-consume
-offset_commit
-error
-partitioner

I think that covers all callbacks in hw-kafka-client.

[AlexeyRaga]

Thanks @phile314 ! Very appreciated!