Mockable functional apis for uni tests

[abose]

There should be a way to simulate mockable APIs with ram stored documents.

We could do something like this:

• put: https://github.com/phcode-dev/extensionService/blob/main/test/unit/utils/sync-test.spec.js#L35
• getFromIndex: https://github.com/phcode-dev/extensionService/blob/main/test/unit/utils/sync-test.spec.js#L53

It might be better to have first-party mock support integrated into the source. For Eg. when initing we could give init(...mockDBMode=true) and the mocks will kick in and can use the simulated in-memory db for unit tests without any code code changes for tests.