Implement Acquire for PgListener

[chanks]

I have a binary that mostly listens to notifications for a few channels, then does some work and writes some results back to Postgres when it's done. PgListener already implements Executor, so it's possible to borrow the listener's connection in order to do this for simple writes (and let this binary only use a single PG connection, since they're somewhat expensive and limited), but there's currently not a way to open a transaction via PgListener's connection. So, I implemented Acquire for it.

The test seems a little silly, I guess it's not catching many issues that the type system doesn't, but it felt reasonable to assert that it's actually possible to open up a transaction from the listener's connection.

Thanks! Feedback welcome!

[joeydewaal]

Looks like someone beat you to it (#3550)

[chanks]

That's true! I'll just close this one out then, thanks!