[web-db] implement setting/getting currentUserID on worker

Summary:
Store currentUserID in SQLite database.

It can be done by different way - performing operations directly on metadata table with parameters, but I wanted to make it similar to CommCoreModuleSchema.

Depend on D7181

Test Plan: Send SET_CURRENT_USER_ID and GET_CURRENT_USER_ID messages to worker and check if it works

Reviewers: michal, tomek, marcin, ashoat

Reviewed By: ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7182