Postgres: Move io to background task.

[joeydewaal]

This PR moves all the io in the postgres driver to a background task. This should fix some long standing issues with cancellation safety and unblock/allow features like query pipelining, CopyBoth mode (#2924) and the ability to cancel queries. This also simplifies the dropping of PgTransaction, PgCopyIn and PgListener. There is still some work to do to increase performance but that's out of the scope of this PR.

Notable changes:

• SocketExt: An extension trait for Socket which has async methods for reading, writing, flushing and closing.
• BufferedSocket now has poll methods for reading messages.
• BufferedSocket has a Sink<&[u8]> impl to send bytes.
• Pipe is added; a temporary stream of responses from the background worker.
• Worker is added; the background worker that handles all the io.
  • The worker now handles ReadyForQuery messages because the wait_until_ready mechanism is removed.
• Shared is added; a shared structure between the connection and background worker.

This is a pretty big change that touches a lot in the postgres driver. I've added a lot of comments to explain my thinking, if there is anything else I can do to make this easier lmk.

Does your PR solve an issue?

TODO

Is this a breaking change?

No, this PR only has breaking changes in sqlx-core which is semver exempt. One unit test had to be changed for PgListener because the buffering mechanism is changed. I don't think that makes this a breaking change and I don't think Hyrum's law applies here.