Create one sync storage post per room

[chriszarate]

Create one sync storage post per room for storing sync updates instead of one singleton post.

Trac ticket: https://core.trac.wordpress.org/ticket/64622

In the base PR, @mindcrtl noted:

One of my first questions, which was already asked by @TimothyBJacobs, is related to the use of a single wp_sync_storage post.

Unless I'm misreading it, when add_post_meta, update_post_meta, and delete_post_meta are called, they invalidate all the cached meta for that post ID, but because every room shares the same wp_sync_storage post, that means all cached sync data across all rooms is invalidated. It's hard to understand why an editing session on post ID 1 should impact the caching and performance of a session on post ID 2.

In testing with 3 clients, I'm seeing thousands of post meta function calls within minutes. I think some of that could be alleviated by giving each room its own wp_sync_storage post, so that meta cache invalidation is scoped to a single session rather than all of them.

This implements that suggestion, creating one wp_sync_storage post per room and storing updates in post meta against each post.

[westonruter]

This could become a const now instead.

[chriszarate]

Done in c90ca32

[westonruter]

In practice, will there ever be more than one ID in this array for a given request?

[chriszarate]

Yes, because all sync requests for a given client are batched into a single request. If you inspect a request payload, you will see something like:

The post IDs are cached because post meta is accessed multiple times, once for awareness state and at least once for sync updates.

[chriszarate]

I've given this some good testing, it's straightforward, and the feedback suggests that is an impactful improvement. Therefore I'm going to merge it into the base branch. Feel free to leave additional feedback on that PR.