Upgrade porsager/postgres library dependency

[specious]

The library that facilitates access to the postgres database from the backend functions written in nodejs ...

littermap-aws-backend/layers/postgres/nodejs/package.json

Line 5 in 1b16f49

"postgres": "^2.0.0-beta"

... has reached a new major version, and so it makes sense to carefully evaluate what has changed and make careful adjustments to the code that relies on it to work with the new release.

In particular, "Breaking changes from v2 -> v3" mentions that queries are now asynchronous.

[specious]

This may become unnecessary if solving #5 removes any need for a PostgreSQL database.