The parallelization requires splitting Msgmap, text+term
indexing, and thread-linking out into separate processes.
git-fast-import is fast, so we don't bother parallelizing it.
Msgmap (SQLite) and thread-linking (Xapian) must be serialized
because they rely on monotonically increasing numbers (NNTP
article number and internal thread_id, respectively).
We handle msgmap in the main process which drives fast-import.
When the article number is retrieved/generated, we write the
entire message to per-partition subprocesses via pipes for
expensive text+term indexing.
When these per-partition subprocesses are done with the
expensive text+term indexing, they write SearchMsg (small data)
to a shared pipe (inherited from the main V2Writable process)
back to the threader, which runs its own subprocess.
The number of text+term Xapian partitions is chosen at import
and can be made equal to the number of cores in a machine.