ci: build pre-release with prepared change log

fix: a restarted follower should not wait too long to elect. Otherwis…

…e the entire cluster hangs

doc: update bug fixed in README

refactor: add defensive check: a PurgedMarker log should never be app…

…lied to state machine

fix: RaftCore.entries_cache is inconsistent with storage. removed it.

- When leader changes, `entries_cache` is cleared.
  Thus there may be cached entries wont be applied to state machine.

- When applying finished, the applied entries are not removed from the
  cache.
  Thus there could be entries being applied more than once.

fix: install snapshot req with offset GE 0 should not start a new ses…

…sion.

A install-snapshot always ends with a req with data len to be 0 and
offset GE 0.
If such a req is re-sent, e.g., when timeout, the receiver will try to
install a snapshot with empty data, if it just finished the previous
install snapshot req(`snapshot_state` is None) and do not reject a
install snapshot req with offset GE 0.
Which results in a `fatal storage error`, since the storage tries to
decode an empty snapshot data.

- feature: add config `install_snapshot_timeout`.

refactor: upgrade logging level for important state changing event

refactor: add logs for every event leader receives.

change: change-membership should be log driven but not channel driven

A membership change involves two steps: the joint config phase and the
final config phase.
Each phase has a corresponding log invovled.

Previously the raft setup several channel to organize this workflow,
which makes the logic hard to understand and introduces complexity when
restarting or leadership transfered: it needs to re-establish the channels and tasks.

According to the gist of raft, all workflow should be log driven.
Thus the new approach:
- Write two log(the joint and the final) at once it recevies a
  change-membership request.
- All following job is done according to just what log is committed.

This simplifies the workflow and makes it more reliable and intuitive to
understand.

Related changes:

- When `change_membership` is called, append 2 logs at once.

- Introduce universal response channel type to send back a message when
  some internal task is done: `ResponseTx`, and a universal response
  error type: `ResponseError`.

- Internal response channel is now an `Option<ResponseTx>`, since the
  first step of membership change does not need to respond to the
  caller.

- When a new leaser established, if the **last** log is a joint config
  log, append a final config log to let the partial change-membership be
  able to complete.

  And the test is added.

- Removed membership related channels.

- Refactor: convert several func from async to sync.

change: MembershipConfig.member type is changed form HashSet BTreeSet