Make accounts independent of mongo

[jamesmillerburgess]

While working on https://github.com/meteor/meteor/pull/9558, I was reminded of this part of the 1.6 announcement about how Apollo and Meteor will be coming closer together.

Accounts is a pretty popular part of Meteor, so to keep it modern in an Apollo-first, MongoDB-independent Meteor 2.x(?), we should be able to use them together. I'm imagining that we add an abstraction layer between accounts and collections and separate out the mongo-specific code into a accounts-mongo package in (version 1.x), later adding an accounts-apollo alternative that can eventually become the default (version 2.0?). This would also leave open the door for other accounts database plugins to be written, if needed.

[toinevk]

@jamesmillerburgess Interesting idea! Just to add some thoughts, would it make more sense to split accounts-base into 3 e.g. Accounts-Base-Client, Accounts-Base-DDP and Accounts-Mongo?

Apollo would have primarily replaced the DDP layer I would have thought. I do like the idea of decoupling Mongo as well to add additional flexibility.