Tweak catalog indexing abstraction for upcoming WARM

Tweak catalog indexing abstraction for upcoming WARM

Split the existing CatalogUpdateIndexes into two different routines,
CatalogTupleInsert and CatalogTupleUpdate, which do both the heap
insert/update plus the index update.  This removes over 300 lines of
boilerplate code all over src/backend/catalog/ and src/backend/commands.
The resulting code is much more pleasing to the eye.

Also, by encapsulating what happens in detail during an UPDATE, this
facilitates the upcoming WARM patch, which is going to add a few more
lines to the update case making the boilerplate even more boring.

The original CatalogUpdateIndexes is removed; there was only one use
left, and since it's just three lines, we can as well expand it in place
there.  We could keep it, but WARM is going to break all the UPDATE
out-of-core callsites anyway, so there seems to be no benefit in doing
so.

Author: Pavan Deolasee
Discussion: https://www.postgr.es/m/CABOikdOcFYSZ4vA2gYfs=M2cdXzXX4qGHeEiW3fu9PCfkHLa2A@mail.gmail.com