[web] implement function to clear database content

Summary:
Sometimes we might want to manually delete file and destruct object in runtime. Otherwise, Emscripten will use reference counting to destruct objects.

Depends on D8553

Test Plan: Tests in D8558.

Reviewers: michal, tomek

Reviewed By: michal

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8611