Invent struct ReindexIndexInfo

Invent struct ReindexIndexInfo

This struct is used by ReindexRelationConcurrently to keep track of the
relations to process.  This saves having to obtain some data repeatedly,
and has future uses as well.

Reviewed-by: Dmitry Dolgov <[email protected]>
Reviewed-by: Hamid Akhtar 
Reviewed-by: Masahiko Sawada 
Discussion: https://postgr.es/m/20201130195439[email protected]