Increased thread id struct to work on 64bit Linux, fixes #524

[mawenzy]

Fixes #524

Increase size of the struct which holds the threads id's, because these are 8 bytes long on 64bit Linux.

This allows the bind_threads option to work on Linux, but will break compatibility with any previous version of rpyc.

In rpyc/core/protocol.py:

 remote_thread_id, local_thread_id = brine.I4I4.unpack(message[:16])
 message = message[16:]

I am not sure why you used 16, since the '!LL' struct should only be 8 bytes long. But this is the right size for the '!QQ' struct

[comrumino]

Thank you for opening the PR. I totally forgot to commit and push some changes from one of my computers.... So, I combined our comments. Again, thanks!