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.
cassandra: updated domainCache comment
Merge pull request #67 from iParadigms/store-fewer-parsed-urls

cassandra: don't add links if not crawling domain