[preview] Cache and share preview snapshots between all subscribers

[bjoerge]

This is a refactor of the preview machinery that enables sharing of preview snapshots among all components that previews fields documents.

Previously, each component/part would have its own version of the document that it previewed, this caused a lot of double fetching, delayed UI updates, etc.

The major change introduced by this PR is that now all parts of the system that requests to preview fields of a document share the same cached representation of the document. For example, if ComponentA wants to preview the fields fieldA and fieldB on document with id doc-123, a cache entry is made for the document with id doc-123, and we fetch and subscribe to changes in fieldA and fieldB of doc-123. Later on, ComponentB may want to preview fieldB, and fieldC on the same document. Now we only fetch and subscribe to changes in fieldC of doc-123. The cached snapshot is shared between both components. If both of these components unsubscribe, we shut down the listeners and no longer update the cached snapshot. However, we still keep the cached snapshot in memory for later. This comes in handy if e.g. ComponentA re-subscribes to the same document/fields. We then immediately emit the last previous known snapshot, while fetching (syncing) the fields again and subscribing to changes in the background.

I also renamed a few functions/files and added a few deprecation warnings to the public API. This API is AFAIK only used by us, and not documented so it shouldn't matter much, and the warnings are there mainly to make it easier to spot our own internal usage.

There is still room for improvements code-wise here, but I've tested this branch quite extensively and feels pretty confident that it works and doesn't contain any regressions (famous last words, I know 😅).

[rexxars]

Looks good. There are a couple of debug console.log statements that's just been commented - should we consider a debug part/module to make it easier to include debugging tools without altering code?

(Not related to this PR specifically)

[bjoerge]

Should we consider a debug part/module to make it easier to include debugging tools without altering code?

Very good point. I created #616 for a discussion about this.