Inefficient deletion of obsolete SwPortVlan records

[lunkwill42]

This method appears to produce SQL statements that become extremely inefficient on large NAV installs (with many, many interfaces):

https://github.com/UNINETT/nav/blob/178616318edf35b7250b889b53f87d09229996fa/python/nav/topology/vlan.py#L409-L411

Most of the time, the same set of switch ports are being updated (or confirmed) in swportvlan, meaning deletions of obsolete records are likely rare. Yet, this method, through Django, can cause an SQL statement of over 500KB in size to be sent to the PostgreSQL server on an install with over 40K switch ports.

The statement produced follows the template

DELETE FROM "swportvlan" WHERE NOT ("swportvlan"."interfaceid" IN (1, 2, 3, 4, .... ));

Which seems to be really inefficiently processed by PostgreSQL (upwards of 30 seconds, resulting in no records deleted on the test installation). It seems that processing the set difference of interface primary keys in Python is a lot more efficient.

Calculating the difference in Python would also make debug logs more readable, since we can print exactly which records to delete, rather then try to parse a 500KB list of interface id's to NOT delete.