VPP-1027: DNS name resolver

VPP-1027: DNS name resolver

This patch is a plausible first-cut, suitable for initial testing by
vcl (host stack client library).

Main features;
- recursive name resolution
- multiple ip4/ip6 name servers
- cache size limit enforcement
  - currently limited to 65K
- ttl / aging
- static mapping support
- show / clear / debug CLI commands

Binary APIs provided for the following:
- add/delete name servers
- enable/disable the name cache
- resolve a name

To Do list:
- Respond to ip4/ip6 client DNS requests (vs. binary API requests)
- Perf / scale tuning
  - map pending transaction ids to pool indices, so the cache
    can (greatly) exceed 65K entries
- Security improvements
  - Use unpredictable dns transaction IDs, related to previous item
  - Make sure that response-packet src ip addresses match the server
- Add binary APIs
  - deliver raw response data to clients
  - control recursive name resolution
- Documentation

Change-Id: I48c373d5c05d7108ccd814d4055caf8c75ca10b7
Signed-off-by: Dave Barach <dave@barachs.net>