Move creation of SparsePackCache identifier string

[jdolence]

PR Summary

This is a tiny PR that just moves the creation of the string identifier for sparse packs into the descriptor. While descriptors can almost always be made static variables (so they only get built once), the packs themselves cannot. Moving the creation of the identifier string to the descriptor means we often only have to build the string once.

It's not a huge improvement, but believe it or not you can actually see a reliable boost in performance from this in our downstream code.

PR Checklist

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md
• CI has been triggered on Darwin for performance regression tests.
• Docs build
• (@lanl.gov employees) Update copyright on changed files

[jdolence]

@pdmullen @Yurlungur @bprather

this is super short. quick approval?

[Yurlungur]

Good improvement 👍