Minor updates to general contacts sync.

Summary:
Add contact UID type assertion.
It's easy to accidentally pass an int rather than a string, but doing so is Bad
News.

In Contact.copy_from, copy fields whether or not they're None.
This is needed to properly sync individual deleted attributes of EAS contact
objects.

Test Plan: ./runtests

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D86