HOLI-10626: use Postgres.js instead of Denos postgres client

Rationale: Denos postgres client does not support automatically closing idle or old connections.
Therefore, as rumors have it, Google Cloud SQl will eventually close the DB connections, leading to
the ECONNRESET error messages we've seen in logging.

This MR is an attempt at avoid ECONNRESET by using a client that supports automatically closing idle
and old connections.