v0.13.0

The second release of the Postgres adapter: the absent database is answered,
a lock is held where it was not, and the durable pairing is renamed.

com.vaelii/postgres is an Apache-2.0 adapter on the SSPL engine. It depends
on core; core never depends on it. open-kb reaches it lazily through
{:backend :pg-memory} or {:backend :pg-disk-log}.

• :pg-disk is :pg-disk-log. The engine reads the second disk in a
  backend name as out of core where it means durable, and renamed both
  pairings that carried it — its own :disk and this one — refusing the old
  spellings rather than aliasing them, since an index axis names a directory
  layout. Nothing about the store, the tables or the directory moves; the
  selector is the whole of it. Migration: {:backend :pg-disk …} →
  {:backend :pg-disk-log …}.
• A source over a database no sink has written answers absent.
  read-manifest on a database with no image tables answers nil — the seam's
  absent case — so a first-run load-index! reports
  {:index :rebuild :reason :absent} instead of raising undefined_table, and
  drop-image! skips a table that is not there, as its docstring says.
• premise-ids fills the strength cache under the fill lock, the read side
  every other fill holds, so a concurrent unmark or delete can no longer be
  reinstated by a reader's walk landing after it.
• A failed sink open releases its connection. pg-sink closes the
  connection it borrowed when construction throws past the borrow, the guard the
  record store's open already carried.

The number. The engine, the plugin and both adapters ship one version
string, checked at the cut. Requires core 0.13.0.