[web-db] add metadata table operations

Summary:
Native version: link;
On the web (for now) we will need this to set current_user_id - to somehow connect database with user, to decide when we should clear sensitive data.

NOTE: D7121 is not landed yet, and even so, my local environment is the only place where this code was executed so adding this without migration.

Depends on D7154

Test Plan: run tests D7181

Reviewers: michal, tomek, marcin, ashoat

Reviewed By: ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7180