Migration for table split

Summary:
ImapAccount -> Account, ImapAccount
Thread -> Thread, ImapThread,
imapaccount_id->account_id in Namespace, Contact

Migration for EAS tables not needed.

Test Plan:
Both upgrading and downgrading tested to work:
To test upgrade, a database dump from a pre-table split sync was used
To test downgrade, the tests/data/base_dump.sql was used)

Reviewers: spang, kav-ya

Reviewed By: spang

CC: mg

Differential Revision: https://review.inboxapp.com/D48