refactor: Remove Item from merger's Node trait #13611

Job	Run time
coverage	10m 49s
Check (windows-latest)	6m 3s
Build GreptimeDB binaries (ubuntu-20.04)	4m 2s
Check (ubuntu-20.04)	1m 37s
Rustfmt	28s
Clippy	1m 58s
Toml Check	21s
Spell Check with Typos	15s
Sqlness Test (ubuntu-20.04)	1m 40s
Compatibility Test	28s
Sqlness Test with Kafka Wal (ubuntu-20.04)	2m 30s
	30m 11s

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