ci: v0.6.4

ci: fix changelog path for 0.6.3

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.