This has some TODOs still but I want to land it now anyway, and fix the
TODOs later.
* Add this.offsetMap which maps each linear model offset to a model tree node
* Refactor createNodesFromData()
** Rename it to buildSubtreeFromData()
** Have it build an offset map as well as a node subtree
** Have it set the root on the fake root node so that when the subtree
is attached to the main tree later, we don't get a rippling root
update all the way down
** Normalize the way the loop processes content, that way adding offsets
for content is easier
* Add rebuildNodes() which uses buildSubtreeFromData() to rebuild stuff
* Use rebuildNodes() in DocumentSynchronizer
* Use pushRebuild() in TransactionProcessor
* Optimize setRoot() for the case where the root is already set correctly
Change-Id: I8b827d0823c969e671615ddd06e5f1bd70e9d54c
