Limit the size of TID lists during parallel GIN build

When building intermediate TID lists during parallel GIN builds, split
the sorted lists into smaller chunks, to limit the amount of memory
needed when merging the chunks later.

The leader may need to keep in memory up to one chunk per worker, and
possibly one extra chunk (before evicting some of the data). The code
processing item pointers uses regular palloc/repalloc calls, which means
it's subject to the MaxAllocSize (1GB) limit.

We could fix this by allowing huge allocations, but that'd require
changes in many places without much benefit. Larger chunks do not
actually improve performance, so the memory usage would be wasted.

Fixed by limiting the chunk size to not hit MaxAllocSize. Each worker
gets a fair share.

This requires remembering the number of participating workers, in a
place that can be accessed from the callback. Luckily, the bs_worker_id
field in GinBuildState was unused, so repurpose that.

Report by Greg Smith, investigation and fix by me. Batchpatched to 18,
where parallel GIN builds were introduced.

Reported-by: Gregory Smith <gregsmithpgsql@gmail.com>
Discussion: https://postgr.es/m/CAHLJuCWDwn-PE2BMZE4Kux7x5wWt_6RoWtA0mUQffEDLeZ6sfA@mail.gmail.com
Backpatch-through: 18

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