[Raft] Add Raft transport

[MariemBaccari]

Following the plan describde in #1463, this PR adds the Raft transport implementation.
It also fixes some import issues.

At this point, it is possible to start a cluster of connected nodes using:

go run ./cmds/core-service \                         
  --store_type=raft \
  --raft_node_id=1 --raft_peers="1=http://localhost:9021,2=http://localhost:9022,3=http://localhost:9023" \
  --accepted_jwt_audiences=dss --public_key_files=build/test-certs/auth2.pem

[the-glu]

Looks good, a few nits :)