plr feature - add support for transactions in procedures

[AndreMikulec]

Add Peter Eisentraut:
See his code.
Add support for transactions in procedures
petere@1b275d1
https://github.com/petere/plr/tree/procedure-transaction

1. Integrate using this code
2. Add test code
3. Add an example to the docs here: https://github.com/AndreMikulec/plr/blob/master/userguide.md

[AndreMikulec]

Done.