Add a generic raft node

To remove beehive's external dependency on etcd, we implement a
generic raft node based on etcd's raft implementation. This code
will be used in the Hive to implement the registry and also in
Bees to implement replication.
Removes bolt db state backend

As we are moving to store everything in raft, we don't need to have a
DB backend.
Refactor code for Beehive 0.2

This is a very large refactor that changes many aspects of beehive:
- Removes etcd dependency and adds raft within the code.
- Moves all go source to the root to be idiomatic.
- Moves independent codes (e.g., state, connection pool, ...) to their
  own packages.
Merge branch 'master' of github.com:soheilhy/beehive