We want to have a single DB object in use for both an app running on top op objectstore, and the sync client itself. Because

 * It is a prerequisite for the webasm effort (which will rely on a single DB object in use)

 * It also eliminates/makes it possible to eliminate code which will help reduce binary size

As part of it, it makes sense to get rid of two components of the sync client:

 * The client file access cache.

 * The changeset cooker

The work is currently in this PR: https://github.com/realm/realm-core/pull/4384