Do optimizations in frequently called RMIs

[jinbpark]

• Some RMI commands (e.g., REC_ENTER) are called lots of times so might be critical to overall performance. Make such jobs as fast as possible. (e.g., a more efficient data structure)

[zpzigi754]

I totally agree with this. I think that making rmm with less exits itself could be a good research direction.

[jinbpark]

FYI: This idea came from @bokdeuk-jeong .