Start the builds in the order of the messages

The queue starts the node whose context ends first: a leaf ends before
its message and a merge after its last message, and at a tie the merge
starts first. With leaves before all merges, a backlog of long messages
kept back the merges that a batch needs, so the view could not shrink.
The merges wait in a heap by the end of their context, so the next build
is found without a scan. The leaf window of 8 unbuilt leaves and the
limit of 8 builds stay as they are.

Issue: beb9a70