centralize versions in top-level POM
isolated protobuf RPC stuff in own module

* barge-api contains base objects (model), currently generated through protobuf but would be better to have POJOs
* barge-core contains the base replica logic and Raft protocole implementation itself
* barge-proto-rpc implements specific replica instance based on Netty/Protobuf client/server
fix wiring of modules and make tests more explicit
further separation of concerns between core and rpc

* moved some interfaces and annotations to api
* make proto-rpc depends on api and not on core
* create NettyRaftService that knits everything together

Integration test still failing but for apparently different reasons than in master.