DocumentHead: Only reset unreadCount on ROUTE_SET

[ockham]

Variation of #25005 (without requiring #24980).

This is the conclusion of a call with @a8dar and @sirreal, based on previous work and investigation found in #25493 and #23961, etc.

Reverts most of #8224, except for its main objective: to reset unreadCount upon route change. However, resetting all the other parts of document-head (title, meta, and links) turned out to be undesirable found documented in the issues and PRs linked from this PR :)

Fixes #24977. Also fixes #23924.

Stealing @a8dar's testing instructions from #25493:

1. Checkout the branch and start the server npm start. You can also do it on calypso.live, but locally you have more control on the cache.
2. Access https://calypso.localhost:3000/log-in
3. See the page title. Should contain 'Log In'. In production probably it doesn't.
4. Look at the page source (not Inspector, but rather real page source that was initially generated). You should see a rel=canonical pointing to the /log-in source.
5. [Skipping step 5 from Canonical URLs: fix /log-in DocumentHead: canonical, title, and meta #25493 which doesn't apply]
6. Access themes section and make sure everything is ok, as in production. Themes section is also SSR-ed and uses the cache. Watch for the page title, hoping that everything works as expected (or better than expected).

Now reload any of these pages, and verify that the site title is still intact, and that the page source continues to include canonical links.

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This finally fixes server-side caching of document-head stuff.
Potential (but less crucial) follow-ups include #24980 and #25003.

[matticbot]

Test live: https://calypso.live/?branch=update/dont-reset-document-head-on-route-set

[ghost]

Tested and looks good, works as expected. Both title and canonical seem to be now consistent on SSR.

I also looked for sections where the title would not be reset when switching the route and found the 'Domains' one (practically when entering the Domains from the left side menu, the old title would persist). We could fix that later.

WARNING: one test seems to have failed, better to check what's happening before merging.

[sirreal]

All tests passing now 💚