change: Require AppData to impl Debug and Display

This enables better debugging and logging by displaying actual application
data in entry payloads, rather than just showing "normal" for all entries.
The change updates Debug/Display implementations to include app data content
and adds corresponding tests.

Upgrade tip:

Implement `Debug` and `Display` for `RaftTypeConfig::D: AppData`

Doc: add change log 0.9.21

fix: reset progress when heartbeat response indicates conflict

With `allow_log_reversion` enabled:

Before this fix, `RaftCore` ignored `conflict` messages from `Heartbeat` RPCs,
preventing leaders from discovering follower state changes. When a follower's
state reverted and responded with a conflict message, the leader wouldn't
retransmit necessary data to the follower.

This commit ensures `conflict` responses are always processed properly and
progress is reset to trigger data retransmission to the follower.

In this commit, a `Heartbeat` message uses `committed` log id as the
`prev_log_id` to detect Follower state reversion.

Doc: add change log 0.9.20

Doc: add change log 0.9.19

feat: Add impl `IntoNodes<NID, EmptyNode>` for `BTreeSet<NID>`

EmptyNode can be created directly thus a BTreeSet can be used to build a
nodes map

docs: add test coverage badge

Doc: add change log 0.9.18

Test: split storage test get_initial_state_membership_from_log_and_sm…

…() into 3 sub tests

Doc: add change-log 0.9.17