reindexdb: Add the index-level REINDEX with multiple jobs

reindexdb: Add the index-level REINDEX with multiple jobs

Straight-forward index-level REINDEX is not supported with multiple jobs as
we cannot control the concurrent processing of multiple indexes depending on
the same relation.  Instead, we dedicate the whole table to certain reindex
job.  Thus, if indexes in the lists belong to different tables, that gives us
a fair level of parallelism.

This commit teaches get_parallel_object_list() to fetch table names for
indexes in the case of index-level REINDEX.  The same tables are grouped
together in the output order, and the list of indexes is also rebuilt to
match that order.  Later during processingof that list, we push indexes
belonging to the same table into the same job.

Discussion: https://postgr.es/m/CACG%3DezZU_VwDi-1PN8RUSE6mcYG%2BYx1NH_rJO4%2BKe-mKqLp%3DNw%40mail.gmail.com
Author: Maxim Orlov, Svetlana Derevyanko, Alexander Korotkov
Reviewed-by: Michael Paquier