Refactor: Snapshot with same last log id can be installed

Refactor: refine method names for `TransferLeaderRequest`

Refactor: ignore notification for Leader/Candidate when node is in su…

…ch state

Fix: (0.10) Clarify that receiving an equal vote does not grant leade…

…rship.

A node's `vote` may be updated when a leader observes a higher vote.
In such cases, the leader updates its local vote and steps down.
However, this vote update does not imply that the node accepts the
higher vote as valid for leadership, as it has not yet compared their
logs.

In this commit, re-enable `VoteResponse.vote_granted` to indicate a vote
is granted.

Chore: mark StorageIOError as deprecated

Refactor: append log entries to local store in non-blocking mode

Since this commit, `RaftCore` returns at once upon submitting
ApendEntries IO request to `RaftLogStorage`, without waiting for the IO
to be flushed to disk. When flushed, the result is responded to
`RaftCore` via a `Notify` channel.

This way `RaftCore` won't be blocked by AppendEntries IO operation:
while entries being flushing to disk, `RaftCore` is still able to deal
with other operations.

Upgrade(non-breaking) tip:

- Deprecated `LogFlushed`, use `IOFlushed` instead.
- Deprecated `LogFlushed::log_io_completed()`, use `IOId::io_completed()` instead.

Fix: install_snapshot() should return local vote, not request vote

Refactor: even when `generic-snapshot-data` is enabled, the old chunk…

…ed transport be still available

If `generic-snapshot-data` is enabled, the old chunk based transport
such as `RaftNetwork::install_snapshot()` for sending and
`Raft::install_snapshot()` for receiving should be still be available
but just deprecated.
This way the application can upgrade Openraft without modification,
except several `#[allow(deprecated)]` attributes.

Feature: add trait `RaftLogStorageExt` to provide additional raft-log…

… methods

The `RaftLogReaderExt::blocking_append()` method enables the caller to
append logs to storage in a blocking manner, eliminating the need to
create and await a callback. This method simplifies the process of
writing tests.

Change: rename `Raft::install_complete_snapshot()` to `install_full_s…

…napshot()`

Rename `Raft::install_complete_snapshot()` to `install_full_snapshot()`;
Rename `RaftNetwork::snapshot()` to `full_snapshot()`;