Rebuilt basic zpipe server using zproto

The goal here is to replace the current hand-written zpipes+zpipes_agent classes by a new generated zpipes_server class. The core of this class is a state machine (zpipes_server.xml) that turns into a generated engine (zpipes_server_engine.h). 

This approach addresses the problem of handling client connections robustly and statefully. The generated server engine tracks client connections, timeouts, etc.