Implement SPI_connect_ext and related functionality
This allows pgx users to use SPI in a non-atomic fashion.
I'm expecting the tests will break on pg10.
Thank you for the feedback; I will address your commentary in the coming days. This is a lower-priority development for me at the moment, but I'll see what I can do.
Another solution to the issues regarding Postgres 10 may just be waiting for a bit: we're going to be dropping Postgres 10 sometime soon, which may be a matter of weeks.
I'll retire this for now, as it has become stale. May reopen later. WIth sub-transactions available through pgx-contrib-spiext, a bit less of a pressing concern.