raftLog: decouple log data structure and flow control

The [raftLog](https://github.com/etcd-io/raft/blob/026484c68b7bc59cd208e216d35e1ff747d24d9c/log.go#L24) structure currently plays multiple roles. Notably:

- Provides read/write [access](https://github.com/etcd-io/raft/blob/026484c68b7bc59cd208e216d35e1ff747d24d9c/log.go#L25-L34) to the raft log (both the `unstable` in-memory part, and the `Storage` part), and keeps the basic metadata about the log.
- To some degree, ensures correctness w.r.t. the core `raft` algorithm.
- Implements [flow control](https://github.com/etcd-io/raft/blob/026484c68b7bc59cd208e216d35e1ff747d24d9c/log.go#L35-L62) mechanisms for applying the commands from this log.

The flow control role is quite distinct from the other roles, and is less fundamental. It should be decoupled. Once decoupling is done, `raftLog` can be isolated in its own package and rigorously tested without assumptions about the flow.

Related to #64, though this clean-up has value on its own.