Support previews and concurrent updates in ce.GeneratedContentNode

GeneratedContentNode didn't track concurrent updates at all, so a
race condition was possible: if the node was updated a second time
before the first update had been rendered, the second update might
render first and then be overwritten by the other one.

To prevent this, we track the promise associated with the current
render. If a new update is launched while a previous one is still
pending we attempt to abort the old one by calling .abort() on it,
and ignore any future resolution or rejection from it.

Also allow rerenders based on non-model data by calling
.update( { config object } );

Change-Id: I8feefd9e8fb6c41d06b8b20131e3be5e37954e83
Merge "Support previews and concurrent updates in ce.GeneratedContentNode"