Stop Conpherence from reloading all messages on reply

Summary:
Currently, when you reply to a Conpherence thread, we load //all// the messages, not just the most recent ones. I don't //think// I broke this, it's just hard to tell that's what's happening unless the thread is very short or you scroll up.

The issue is that we're loading //threads// after some ID, but actually need to load //messages// (transactions) after that ID. Also, the way "before" and "after" work is sort of weird so this actually needs to be "before" (we should probably invert the meanings). Ref T2421.

Test Plan: Replied to a very short thread; saw no duplicate messages.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2421

Differential Revision: https://secure.phabricator.com/D5537