postgres: atomic read message

[rich-murphey]

Here is an attempt to address #886. It peeks at the message header, then consumes the message, all or none.

This looks very awkward compared to the way it's done in the 'next' branch. As a result, I'm not entirely sure whether it may have a performance impact.

This is just an idea for the sake of discussion, so feel free to close this if it's not a preferred way of approaching the issue.

[rich-murphey]

I think there may be an off-by-one error somewhere in this PR. While testing dropped streams, I'm seeing:

 sqlx_actix_streaming::bytestream:dropped ByteStream in state: NonEmpty after 71 items
 sqlx_core::pool::connection:error occurred while testing the connection on-release: encountered unexpected or invalid data: encountered unexpected or invalid data: unknown message type: '\"'

[rich-murphey]

I'm closing this because additional effort would be better spent on the 'next' branch.