Add PSQLFrontendMessageDecoder

[fabianfett]

Motivation

In tests we would like to use EmbeddedChannel, if we decode and encode in the PSQLChannelHandler using the NIOSingleStepByteToMessageDecoder, we need to decode messages the handler wrote to the channel from bytes.

Changes

• Add PSQLFrontendMessageDecoder
• Add ReverseByteToMessageHandler

Notes

The changes are mostly in the test target. Enabling easier tests in the future.

[gwynne]

This and the other guards on readableBytes are redundant; getInteger(at:as:) already does this:

        guard let range = self.rangeWithinReadableBytes(index: index, length: MemoryLayout<T>.size) else {
            return nil
        }

So for that read you can just check that result for nil instead of having a separate check. All other read*() and get*() methods also do this, so you can simplify and get rid of the !s.

This all being said, I very, very vaguely once recall there possibly having been a reason not to do that... I have no idea what it was or if it was a real thing and not my fevered imagining, but if so, please tell me 😰

[gwynne]

Just the one commentary on dealing with the ByteBuffer, if there's a reason to leave it as is I'm not gonna try to argue the point 🙂

[gwynne]

LGTM