ZOOKEEPER-2623: [ADDENDUM] Forbid OpCode.check outside OpCode.multi

[kezhuw]

Individual OpCode.check will get UnimplementedException.

JIRA: ZOOKEEPER-2623
Discussion thread: https://lists.apache.org/thread/vl816jfrklvqz29coz5qnwpom9q41pcg

This pr reworks/reverts #1988, which return an empty response for individual OpCode.check, for the following reasons:

• The individual OpCode.check is covered by OpCode.exists.
• In certain situation, we may want to differentiate the two(individual or not), say statistics. This could cause maintenance burden for us.
• Exposing opcode without explicit API is probably not a good.

Open this pr for easy evaluation. I will reply the discussion thread for reference.

I plan to open a draft backport on branch-3.9 to ease code review if we are willing to go this direction.

[kezhuw]

I have opened #2105 onto branch-3.9 without commits from #1988 so to make it clean for reviewing.