Track the root of a post tree explicitly

Querying the TreeRepository from the TopicHistoryIndex is open
to slave-lag issues, the just-added post may not actually
be available in the slave yet.

Anything we are saving we already know the root for, we just didn't
have the information available in the TopicHistoryIndex.  This patch
adds another field, rootPost, which is set when loading the posts. That
allows the information to make it into the TopicHistoryIndex
and remove the need for the TreeRepository there.

Bug: 58996
Bug: 59803
Bug: 59804
Bug: 59642
Change-Id: I23854ac0e36d8b892eba0d1202c1fd81dc05bd6e