Proxy: copy bidirectional fork (#6720)

## Problem

`tokio::io::copy_bidirectional` doesn't close the connection once one of
the sides closes it. It's not really suitable for the postgres protocol.

## Summary of changes

Fork `copy_bidirectional` and initiate a shutdown for both connections.

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Co-authored-by: Conrad Ludgate <conradludgate@gmail.com>

proxy/src/proxy.rs

@@ -2,6 +2,7 @@
 mod tests;
 
 pub mod connect_compute;
+mod copy_bidirectional;
 pub mod handshake;
 pub mod passthrough;
 pub mod retry;