Upstream awaits each statement of a write batch before building the next,
which costs nothing on a single node and one network round trip each on
the cluster; a batch of a few hundred operations held the account's
change-id row locked for all of them. Statements that need no answer are
now sent as soon as they are built and their replies read later, in
order; PostgreSQL still runs them in the order sent. Operations that need
a reply to continue (asserts, merges, AddAndGet) drain the queue first,
so the first error reported is the real one. The change-id bumps for the
accounts in a batch go out together too.
Verified with the tests crate's store suite against PostgreSQL.
jcoffey-dev