Resolve "TODO: tiered" comments in code

This is a catch all for resolving TODO comments added during PM-2119.  These generally have the format: "TODO: tiered: ...".  Some such comments and proposed resolutions are called out in WT-7470, WT-7471, WT-7472.  There are a few others that should be resolved during WT-7378.  Another one, having to do with shared placement of files with WiredTiger prefixes, is specifically mentioned in Appendix E of the "long design doc", so it won't be forgotten.

This ticket is to make sure there is resolution, tickets, or some other path forward on any other such TODOs that fall through the crack.