Create BDR's truncate triggers as 'tgisinternal'

Create BDR's truncate triggers as 'tgisinternal'

Switch from creating triggers using the SPI to using CreateTrigger
directly, marking the new trigger as `tgisinternal` so it's not
dumped by `pg_dump` and not listed by `psql`.

Existing triggers are not updated. A subsequent commit will add
support for updating existing triggers on extension update.

Fixes issue #62