compaction: clean up runCompaction

In this change we separate the code that deals with the "data" portion
of the compaction from the code that deals with the "metadata"
portion. These were intertwined in the old code, leading to a lot of
clutter.

We move the "inner" loop of the compaction (writing one output table)
to `compact.Runner`. We then use the tables in `compact.Result` to
create the version edit.