Seperate synchronization APIs from data access APIs

[jingtang10]

Describe the issue to be researched
The APIs for local resources and sync are quite tightly coupled at the moment. The FhirEngine interface definitely doesn't follow the single-responsibility principle.

Describe the goal of the research
Find out if there's a way to decouple the core storage APIs and sync APIs.

Describe the methodology
Prototyping. Researching other similar codebases.

Would you like to work on the issue?
Yes :)

[jingtang10]

I will continue to work on the draft PR but instead of creating a separate module to host the synchronization APIs, we will keep them in FHIR engine but make sure they are decoupled from the database access APIs.

[jingtang10]

This is done.