feat: let pool.query pipeline on a connection that is already working

[nigrosimone]

close #3738

Passing pipeline: true to a Pool today changes nothing for pool.query(): every query checks out a connection for itself, so there is never a second query to pipeline behind it.

With this change pool.query() can send a query on a connection that is still waiting for results. The pool opens connection up to max first, then picks the one with the fewest queries in flight, capped by a new maxPipeline option (default 10) and by socket backpressure. max keeps its meaning of number of connections. pool.connect() still checks out a connection nobody else is using, so transactions are not affected. Off by default.

A connection with queries in flight is not idle: idle timeout, maxUses, maxLifetimeSeconds and pool.end() wait for the pending results before closing it.

usage:

const pool = new Pool({ max: 10, pipeline: true, maxPipeline: 100 })
await pool.query('SELECT $1::int', [1]) // same call, can go on a connection already working

Disclaimer: made with the help of Opus 5 and a final check with Fable 5. I have done my best to verify that everything is correct and not AI slopped! tested on my side (patching node modules), seem works

[nigrosimone]

A good point from @charmander #3738 (comment)