v0.15.4

Faster bundle imports. A backfill spent nearly all its time sleeping: it paces at half
the lower of the namespace and principal rates (25 items/s at default limits), 40 s per
1,000-item batch, against about half a second of work.

• Under an allowance (§6.6) the importer now paces at the allowance's full rate, since its
  bucket holds up no other writer, and splits batches by its itemsPerBatch/batchSize. It
  finds its own allowance as the core does (the grant's root sub and kid, or the author
  with authentication disabled), counts every draw (dry runs, uploads, copies) so it doesn't
  run into 429s, and from a minute before the allowance's until goes back to the
  namespace's limits. With an allowance of 1,000 items/s, 2,000 documents wait 3.5 s instead
  of 80 s; 145,000 would take minutes instead of over an hour. Without one, pacing is as
  before, less the time each batch took. Progress lines say what the import is paced by.
• Planning reads target heads from /heads pages instead of one GET per document
  (twice in snapshot mode): 0.005–0.014 requests per document instead of 1–2. A small import
  into a large namespace keeps per-document lookups.
• BenchmarkImport (internal/bundle) measures an import's work, requests per document and
  the time it would sleep.

The client's requests use a connection pool of their own, a clone of
http.DefaultTransport, so code that closes the default one's idle connections doesn't
break them.

Full Changelog: v0.15.3...v0.15.4