perf(pg-pool): optimize client retrieval from pool by prioritizing prepared client

[cesco69]

see #3397

Instead of retrieving the first available client from the pool, 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.

[charmander]

Needs tests.

[brianc]

I'm not convinced this is a fair test. In fact, I should check that the query was executed with the same client, but how can I tell if it's the same client?

You can call pg_backend_pid() in a query - that should return the connection ID which will certainly be the same across queries on the same client.

[cesco69]

I'm not convinced this is a fair test. In fact, I should check that the query was executed with the same client, but how can I tell if it's the same client?

You can call pg_backend_pid() in a query - that should return the connection ID which will certainly be the same across queries on the same client.

Thanks! done with 20e7fa2