rpc: add main function for raft server.
rpc: add main function for raft server.

The main function for setting up a raft rpc server is now fixed. It
firstly creates a raft service (RaftServiceImpl::New), which is later registered
to the rpc server. And the server runs infinitely waiting for terminate signal.

neverchanje/yaraft-wal,neverchanje/consensus-yaraft,neverchanje/yaraft-wal,neverchanje/consensus-yaraft,neverchanje/consensus-yaraft,neverchanje/yaraft-wal,neverchanje/consensus-yaraft