zmqpp::socket::receive_raw also has int length

As with my previous pull request for send_raw, the buffer for receive_raw has an
int for length rather than a size_t. I added unit tests for testing sending >2GB
messages using zmqpp::socket and now something failings inside ZMQ. It returns
with EAGAIN.
Removed conflicting changes.
Merge branch 'develop' of https://github.com/cmdevries/zmqpp into develop