Implement GlobalDbSingleton API for both platforms

Summary: Implements GlobalDBSingleton API for both platforms. On iOS we use main queue and atomic boolean for synchronization. On Android we enable multithreading from the begining.

Test Plan: Spawn multiple threads in AppDelegate (for instance multiple instances of WorkerThread) and schedule database tasks from them using GlobalDBSingletonIOSProxy API. Do it before and after CommCoreModule instantiation. No crash should happen.

Reviewers: tomek, atul, jon

Reviewed By: tomek

Subscribers: jon, ashoat, abosh

Differential Revision: https://phab.comm.dev/D5233