HOT WIP Patch - Version 4.4

Please see the version 4.4 of HOT WIP patch posted on pgsql-patches.
I have fixed couple of bugs in the earlier version posted. Other than
that there are not any significant changes in the patch.

The row-level fragmentation had a bug where we were
unintentionally sorting the line pointers array more than
once. Also, the defragmented lengths were computed wrongly
and was a source of many errors.

Another bug fix was in the heap_hot_fetch() code path where
we were asserting on finding a LP_DELETEd tuple in the hot
chain. I later realized that this is not required and we
should rather just assume that the chain is broken, something
very similar to the xmax/xmin checks.

Thanks,
Pavan

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