Provide `needAllTransactions()` for ConpherenceThreadQuery

Summary:
Conphrenece is pretty slow right now; one reason is that threads can not be loaded without also loading all of their transactions. I want to get rid of this requirement, so make it explicit and then make all existing queries require it.

In particular, loading a page like `/conpherence/1/` means we load all the transactions four times: to check that the thread exists, to build the thread list (which also loads all transactions for all other visible threads), to build the thread itself, and load all the transactions to build the widget panels.

Ref T2421.

Test Plan: Viewed threads, lists, widgets; replied to threads.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2421

Differential Revision: https://secure.phabricator.com/D5509