Clean up dragonboat dependency

In the latest decision, we would like to pivot the etc/raft library as a replacement for dragonboat. Those two libraries are absolutely different in terms of philosophy and design. Also, as the project switched the direction to bottom-up, we'll start by injecting etcd/raft directly into the WAL transaction manager. Thus, it makes a lot of sense to remove components built based on dragonboat at the moment. It doesn't make any sense to keep two of them co-exist at the same time. Of course, we don't want to throw everything away.

The history is preserved in Git so that it's trivial to cherry-pick necessary components from the history later.