BTreeMap: Explore making node loading less expensive

[ielashi]

Loading is a relatively expensive operation, and improving it can lead to significant gains in performance. Caching is an obvious way to make loading a node cheaper. Maybe there are other options worth exploring too.

Related forum post: https://forum.dfinity.org/t/is-there-any-way-to-mutate-the-value-inside-ic-stable-structures-btreemap/