[web] implement functions to import/export database content

Summary:
This is using Emscripten virtual file API.

Gets/puts file content on which the native module operates.

Only worker (one thread) operates on file using synchronous API - there should be no problems with locks/parallel access.

Test Plan: Tests.

Reviewers: michal, tomek

Reviewed By: michal

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8559