Automatically enable Condstore for servers who support it

Summary:
I needed this patch to test D297. It does two things:

1. Detect and store condstore support when running inbox-auth
2. When starting a sync choose the right monitor class

Test Plan: Tested by running a full sync + polls on a yahoo and fastmail account. Used the ImapClient logs to check that the right commands where issued.

Reviewers: charles, spang

Reviewed By: spang

Subscribers: mg

Differential Revision: https://review.inboxapp.com/D302