Storage Attached Indexes (Phase 3)

As 5.0 moves toward GA, this epic will serve as a place to organize our work around SAI for the next major release, which I'll call 5.1 for now.

New features, performance improvements, and usability enhancements are all on the table, and it is very likely that at least minor enhancements to the on-disk format will occur. This will bring the versioning hooks (see OnDiskFormat and friends) we've worked on to this point into focus.