refactor(clustering/sync): improve readability and debugability

[StarlightIbuki]

Summary

1. abstract functions and reshape the code, including error handling
2. extra call to notify CP the state of sync;
3. commit DB purge before full sync;
4. more debuging logs;
5. sync once retry will not count timeout as retry, allowing DP to sync deltas right after a full sync

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

KAG-6177

[chobits]

nice!! all my comments are included in its associated EE pr, which have been resolved by xuming already

[chobits]

Not a block, i know you're struggling with this PR.

Suggested change

	if is_full_sync then
	local entity_not_found = err and err:match("MDB_NOTFOUND")
	-- full sync needs extra tolerance for errors
	if not res and not entity_not_found then
	return nil, err
	end
	else
	if not res then
	return nil, err
	end
	end
	if is_full_sync then
	local entity_not_found = err and err:match("MDB_NOTFOUND")
	-- full sync needs extra tolerance for errors
	if not res and not entity_not_found then
	return nil, err
	end
	end
	if not res then
	return nil, err
	end

[StarlightIbuki]

They are different. The later is incorrect.