Hello, Matthias and others!

Updated WIP in attach.

Changes are:
* Renaming, now it feels better for me
* More reliable approach in `GlobalVisHorizonKindForRel` to make sure we have not missed `rd_safeindexconcurrentlybuilding` by calling `RelationGetIndexList` if required
* Optimization to avoid any additional `RelationGetIndexList` if zero of concurrently indexes are being built
* TOAST moved to TODO, since looks like it is out of scope - but not sure yet, need to dive dipper

TODO:

* TOAST

* docs and comments

* make sure non-data tables are not affected

* Per-database scope of optimization

* Handle index building errors correctly in optimization code

* More tests: create index, multiple re-indexes, multiple tables

Thanks,
Michail.