Condstore support for MobileMe

Summary:
Briefly, there was a couple bugs with the mailsync code:

1. Some backends need to receive a condstore-enabling message to activate condstore. This is done using a "STATUS CONDSTORE" command.

2. MobileMe sends HIGHESTMODSEQ values bigger than Integer. I changed the db field to be a BigInteger instead. I made the same change to UIDVALIDITY because it's probably the same.

Test Plan: Ran a sync with MobileMe, added a gmail account, checked that condstore worked for both.

Reviewers: emfree, charles

Reviewed By: charles

Projects: #inbox-sync

Maniphest Tasks: T311

Differential Revision: https://review.inboxapp.com/D352