mysql-srv: Optimize buffer use reading from the client socket

[jasobrown-rs]

Description

When looking at a flamegraph produced while running a system-benchmark, I discovered that we spend a lot of time in PacketReader::next() just resizing the backing Vec<u8>. There's more efficient ways to handle the buffer here. Naively, we should look at the bytes crate as it integrates with tokio natively.

This code in PacketReader is fairly self-contained so any optimization should be rather straight forward.

Change in user-visible behavior

N/A

Requires documentation change

N/A