v0.17.1

Fixed

• A scaled world left the identity sequence pointing at rows that came back,
  a regression introduced by 0.17.0 and found by the consumer that prompted it.

  0.17.0 made scaled_world empty the tables its shape declares, so a session
  world could sit under one, and the rows come back because the emptying happens
  inside the transaction it rolls back. The sequence does not: setval is
  not transactional, so the counter kept whatever the scaled build moved it to.
  A scaled world is usually smaller than the session world it was built over,
  which left the counter below the ids that had just returned.

  The symptom was an IntegrityError on a primary key, in a later test, for a
  row the failing test never wrote.

  The sequences are now recomputed once the transaction has ended -- from
  max(pk) of whatever actually survived, rather than from a number captured on
  the way in, so it is correct in both directions and correct too when the
  caller's block raised partway through the build.

  The Postgres statement-count constant moved from 17 to 19: one reset per
  declared table, a property of the declaration rather than the factor, so what
  those tests pin is unchanged. The portable constant did not move, because
  Django emits no sequence reset for a SQLite table without AUTOINCREMENT.