v0.15.2

Performance, follow-ups from the v0.15.1 review:

• Branch /heads pages looked up the branch's base namespace once per name that reads
  through. A read transaction now reads each namespace row once: a branch page takes 27–33 ms
  instead of 161–164 ms on Postgres (a branch of a branch, 34–38 ms instead of 288–297), and
  20–29 ms instead of 36–38 ms on SQLite.
• The index's -fetch-concurrency now limits fetches across all followed namespaces
  and branches together, not per namespace: -ns a,b,c sent up to 24 GETs at once to the
  core (whose Postgres pool has 16 connections) and now sends at most 8. It must be at least
  1.
• The index reuses its connections to the core: its client keeps an idle connection per
  fetch (new client.WithIdleConns), where http.DefaultTransport kept 2 and redialled the
  rest after every page. Catching up 3 × 2,000 documents opened 8–12 connections instead of
  1,500–2,100, at the same speed.

Full Changelog: v0.15.1...v0.15.2