This caches a partition map per DB, which is helpful because it exposes
what the point in time DB partition state is, but is unhelpful because
the partition state can advance underneath the DB cache. This is
generally true of the approach -- this can happen to the ident/entid
maps, and the datoms themselves -- so we'll roll with it for now.
This reduces the number of SQL UPDATE operations from linear in the
number of id-literals used to constant in the number of known
partitions.
This was a little more tricky than might be expected because the
initialization process uses the transactor to bootstrap the database.
Since Clojure doesn't accept mutually recursive modules, this
necessitated a third module, namely "db-factory", which uses both "db"
and "transact". While I was here, I started an "api" module, to paper
over the potentially complicated internal module structure for external
consumers. In time, this "api" module may also grow CLJS-specific JS
transformations.
