Allow Gmail threads to be split among multiple Inbox threads.

Summary:
This is a short-term fix for an uncommon situation that nonetheless breaks
sync. See T599 for details.

Test Plan:
Trigger the breaking condition by sending multiple identical emails;
check that they're reconciled correctly, and that replies get threaded with the
message they're replying to.

Reviewers: spang, kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D713