cassandra: don't add links if not crawling domain

- If Config.AddNewDomains is off, don't add parsed links into the
  database at all
- Made the domain cache apply for existence and non-existence of domains

Considered also not adding parsed links when the domain is marked as
excluded, but since sometimes an exclusion may be temporary I figured
for now we don't need the complexity in there.
Not critical, but maybe 

value is a boolean --> value is a boolean which is true if the domain is in the datastore

or similar.