minimize memory allocations in sanitize

[vtolstov]

I'm using pgbouncer with pgx and saw many memory allocs in internal/sanitize packages.
Did you mind to use simple buffer pool for this https://github.com/jackc/pgx/blob/v4.18.3/internal/sanitize/sanitize.go#L29

So you can use already allocated buffer and reuse it all the time, so we can minimise garbage collector cpu time and also minimise memory usage.

[jackc]

v4 is not getting any changes except bug fixes.

[vtolstov]

What about v5 ?

[jackc]

Yes, it would be reasonable for v5.

[ninedraft]

If no one minds, I'd like to address this issue.

[vtolstov]

@ninedraft if you have free time this will be very helpful.