Postgres support part way in.  However, it does not support a
nice Map<String, Value> class to handle things easily without
defining a mapping struct in Rust.

Switched around to pass around the connection pool from which to
get a new connection. rather than a pooled connection already
pulled off the pool.

Factored out the sqlite functionality into functions below main,
implemented command line parsing, and stubbed in postgres as a 2nd
database.  Postgres implementation is not started yet, just stub
entrypoint for it to be fleshed out later.

Testing out some thing on how Rust handles deserialization from db
rows into JSON.