Move IO to separate thread

[drmingdrmer]

IO operations such as RaftStorage::append_to_log() and RaftStorage::apply_to_state_machine() are currently executed within the tokio-task that runs RaftCore, causing every IO operation blocks the entire system.

To avoid this issue, IO operations should be performed in a separate tokio-task, allowing RaftCore to receive IO completion events via the event channel. This approach will enable RaftCore to handle other requests while IO operation is in progress.

• Add generic parameter Ent: RaftEntry to Engine. #728
• Remove Command::MoveInputCursorBy; Command::AppendInputEntries implies consuming the input #733
• Refactor: replace Vec with VecDeque for Engine output commands type #736
• Refactor: add input_entries to RaftCore #737
• Progress only tracks persisted log id. It should also track transient log id: written but not yet flushed.
• Engine consume input entries and output them into commands #731
• Feature: new API RaftStorage::append_to_log_cb() #735

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• To prevent reference sharing, extract the entries from input entries buffer instead of directly referencing them. The buffer used for input entries can not be shared between RaftCore and the IO-worker.
• IO and network operations are queued in Engine.output.commands. Send these commands to the io-worker thread.

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