Log committed message count for all providers.

Summary:
This lets us easily measure raw sync throughput for all accounts, not just
Gmail ones. Fixes T408.

Test Plan: Run Gmail and Exchange syncs and check that the logs look good.

Reviewers: kav-ya

Reviewed By: kav-ya

Maniphest Tasks: T408

Differential Revision: https://review.inboxapp.com/D437