Build the nodes from a queue in the background

The core keeps the nodes that are ready for a model build in a queue and
never scans the tree for work. A leaf joins the queue when its message
is too long to be its own node. A merge joins when both of its children
are built and their texts do not fit in one node. A parent that fits is
built at once, up the tree.

At most 8 builds run at once. A leaf starts once fewer than 8 leaves
before it are unbuilt. Leaves start before merges, oldest first, because
a turn waits for the leaves of all earlier messages. A failed build
starts again at the next message. Each build gets its compaction view
and its input, the source of a leaf or the two child lines of a merge.

The model stays behind a small Summarizer interface, and the tests use a
fake one. The start reads the log and the tree once to fill the queue
again, and builds the free nodes that a crash left out.

Issue: beb9a70