Housekeeping: log path, dead code, and informal handshake fields

[iTranscend]

1. Hardcoded, relative log file path. The log path is the hardcoded relative string "thala-node.log", writing to the current working directory regardless of --data-dir. A 1.7 MB log file currently sits in the repository root.

2. Dead code. src/tracing_config.rs is completely empty (0 bytes) — tracing config actually lives in shared/src/tracing.rs. Separately, Node::_task_queue: VecDeque<TaskId> is declared but never read or written; the leading underscore only silences the warning.

3. Informal message fields on handshake structs. ConnectionReq and ConnectionResp each carry an Option<String> message used only for debug strings like "Sup peer" and "Sup peer at 127.0.0.1:5674". These serve no protocol purpose and waste serialized bandwidth.

Locations

• src/main.rs, lines 20–27 — hardcoded log path.
• src/tracing_config.rs — empty file to delete (and remove any mod tracing_config; declaration).
• src/node.rs line 99 — unused _task_queue field.
• src/message.rs — ConnectionReq, ConnectionResp, lines 13–28 · src/node.rs line 275 · src/node.rs line 385 — message fields and construction sites.

// src/main.rs — hardcoded relative log path
let tracing_config = TracingConfig {
    enable_file_logging: true,
    file_path: Some("thala-node.log".to_string()),
    ...
};

// src/node.rs / src/message.rs — informal message fields
message: Some(format!("Sup peer at {}", peer_addr)),
message: Some("Sup peer".to_string()),

Fix

• Default the log file path to {data_dir}/thala-node.log, resolved after CLI args are parsed, and add *.log to .gitignore.
• Delete the empty src/tracing_config.rs (and its mod declaration) and remove the _task_queue field and its initializer.
• Remove the message field from both handshake structs and all construction sites.