descs: use nstree.MutableCatalog for uncommitted layer #106151
Description
TFTR! Moving to an `nstree.MutableCatalog` seems reasonable, though I'm not familiar with the data structures in this code, so I'll leave more invasive refactors for the experts. For now, I'll just land this smaller micro-optimization to avoid the 4 heap allocations per txn.
Originally posted by @nvanbenschoten in #105981 (comment)
Jira issue: CRDB-29428
Epic CRDB-58150