add bigserial pk support

This is essentially a backport of the bigserial support I put in master.  It's safe to use our app against this after it's merged, but I want to move our pk sequence stuff out before we update the app to point at this.
Yep.  It's ok though.  This is the same thing we do for uuid on master.  I guess the idea is that if other adapters want / can implement, they will.