Provide retrieving raw connection from connection/transaction object

[maybephilipp]

I want to insert rows with pg-copy-streams, but it requires a support of raw object passed to it. I have a complex transaction in place where I want to use it, so it doesn't actually make sense to switch completely to native pg in it.

The solution I see would be to get a raw connection object from slonik's transaction and use it with pg-copy-streams to insert large amount of rows with streaming.

Related issues:
#161