optimize lookups in snapshot [sub]xip arrays

Hi hackers,

A few years ago, there was a proposal to create hash tables for long
[sub]xip arrays in snapshots [0], but the thread seems to have fizzled out.
I was curious whether this idea still showed measurable benefits, so I
revamped the patch and ran the same test as before [1]. Here are the
results for 60₋second runs on an r5d.24xlarge with the data directory on
the local NVMe storage:

writers HEAD patch diff
----------------------------
16 659 664 +1%
32 645 663 +3%
64 659 692 +5%
128 641 716 +12%
256 619 610 -1%
512 530 702 +32%
768 469 582 +24%
1000 367 577 +57%

As before, the hash table approach seems to provide a decent benefit at
higher client counts, so I felt it was worth reviving the idea.

The attached patch has some key differences from the previous proposal.
For example, the new patch uses simplehash instead of open-coding a new
hash table. Also, I've bumped up the threshold for creating hash tables to
128 based on the results of my testing. The attached patch waits until a
lookup of [sub]xip before generating the hash table, so we only need to
allocate enough space for the current elements in the [sub]xip array, and
we avoid allocating extra memory for workloads that do not need the hash
tables. I'm slightly worried about increasing the number of memory
allocations in this code path, but the results above seemed encouraging on
that front.

Thoughts?

[0] https://postgr.es/m/35960b8af917e9268881cd8df3f88320%40postgrespro.ru
[1] https://postgr.es/m/057a9a95-19d2-05f0-17e2-f46ff20e9b3e%402ndquadrant.com

--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com