[sync] Change the way invalid accounts are handled in mailsync.

Summary:
Previously we had a weird short-circuit for dealing with invalid
accounts in getting a crispin pool. Re-factor how we handle invalid accounts
there as well as when creating a new connection from the crispin pool.

Fixes T410

Test Plan: Run regression + 'self send'.

Reviewers: emfree

Reviewed By: emfree

Projects: #inbox-sync

Maniphest Tasks: T410

Differential Revision: https://review.inboxapp.com/D443