Postgres upgrade should work with orphaned content/metadata rows

[dom]

The upgrade from the old database schema to new should delete any orphaned content/metadata rows in a similar way to the MySQL upgrade (possibly printing a warning).

Otherwise upgrading a broken database will result in

DBD::Pg::db do failed: ERROR:  column "node_id" contains null values

[anonymous]

Throw a warning, then after setting the node_ids on everthing with a node, do

DELETE FROM content WHERE node_id IS NULL; DELETE FROM metadata WHERE node_id IS NULL;

[dom]

(In [317]) Delete orphaned content/metadata rows in Pg setup (closes #10)