implement ZMTP 2.0 (http://rfc.zeromq.org/spec:15) and simplify handling to connect greeting

The original implementation was trying to follow 13/ZMTP as much
as possible. It turned out, that 13/ZMTP is wrong/misleading in
several places. Using informations from 15/ZMTP and network traces
leades to much more simplified logic for handling the greetings.
fix test suites for ZMTP 2.0

The test suites was using invalid socket combinations and the tcp
fragmentation test had an invalid greeting.
add ZMTP version selection and expand test suites to test all version combinations