feat(pg-pool): Optimize client retrieval from pool by prioritizing free client with cached prepared statements

[cesco69]

Hi,

Instead of retrieving the first available client from the pool, would it be possible to first check if there is a free client that already has cached prepared statements? This could improve performance by reducing the need to re-prepare statements on different clients.

I have made a PR https://github.com/brianc/node-postgres/pull/3398