Implement local override of ASN descriptions

[coolparadox]

Sometimes the ASN description returned by third party databases is not optimal.

This is a request to make geoipdb capable of maintain a database of local descriptions of given ASNs.

• These descriptions would take precedence as return values of LookupAsn().
• There must be API endpoints for adding and removing ASN descriptions.

[sajal]

I suggest using geoipdb collection in dnsdist database. The propose following object format :-

{"_id": "AS555", name: "The description"}

The geoipdb lib would take a *mgo.Collection during init, and if not nil then it will do the lookups against mongo.