make unit tests pass again
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
remove gradle related build artifacts