[web-db] get client database store from SQLite

Summary:
Fetching user store (only drafts for now), matching native version.

Depends on D7153

Test Plan:
Run this code

await webDBWorker.scheduleOnWorker({
   type: workerRequestMessageTypes.GET_CLIENT_STORE
});

and check if drafts are properly returned.

Reviewers: michal, tomek, marcin, ashoat

Reviewed By: ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7154