Optimize sitemap

[RealOrangeOne]

Fixes #1698

This PR encompasses a few small optimisations:

• Use .specific(defer=True) when getting pages. This means there's no need to fetch the specific page models at all, removing the extra queries which comes with it.
• Use the DB to calculate the latest lastmod, rather than looping through all pages.
• Use a slightly simpler / faster way of determining the last modified date when looping through pages (copied from Django)
• Cache the Wagtail site on the view

The query drops in the tests is deceptive. These changes dropped them by 2 in the tests - the rest are from using a non-database cache.

With ~10000 pages (averaged over 10 requests using gunicorn and oha):

Before:

Slowest:      1.4849 secs
Fastest:      1.0741 secs
Average:      1.1292 secs
Requests/sec: 0.8856

35 DB queries

After:

Slowest:      1.8591 secs
Fastest:      1.4128 secs
Average:      1.4964 secs
Requests/sec: 0.6683

18 DB queries

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I'm not sure why the updated version appears slower - although I think it's within the margin of error. It's running fewer queries, and defer=True should make the codepaths much simpler, too. The tests were run on bakerydemo and I suspect with more than just a single model the improvements may be greater still.

[squash-labs]

Manage this branch in Squash

Test this branch here: https://realorangeoneoptimize-sitemap-nule6.squash.io

[RealOrangeOne]

The cause of the slowdown seems to be related to the use of .specific(defer=True). As this is the main optimisation, I'll do some digging as to why it's so much slower.

[RealOrangeOne]

The issue is indeed from .specific(defer=True), particularly when used as a queryset (as opposed to a singular use of .specific_deferred). I have a rough fix, but it'll pollute this PR too much, so I'll open it as a separate one. This shouldn't be merged until that's in.