postgresql: add support for cancelation

[Bogdanp]

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This adds support for canceling Postgres queries according to 53.2.8. Canceling Requests in Progress. I don't love the connect&attach stuff, but I wanted to minimize the number of changes, and the ssl-context needs to be carried forward in case the server only accepts TLS connections. I don't feel strongly about adding the postgresql-cancel procedure, and would be happy with just the new cancel method on postgresql-connection<%> for now if you feel that eventually you might want to add a more general connection-cancel procedure that works on other connection types as well down the line.

[Bogdanp]

👋 ping

[Bogdanp]

Bump

[Bogdanp]

Hi @rmculpepper, any thoughts on this change?

[rmculpepper]

I've gone back and forth about the postgresql-cancel procedure, and at the moment I'd rather leave it as just a method. I'll merge this and to the update myself.

[Bogdanp]

Thanks!