Implement DNS cache support

[lucaderi]

This is to request to implement

• DNS cache created decoding DNS traffic (A and AAAA traffic) whose entries respect the DNS traffic TTL, with some upper-bound values to avoid exhausting all the resources
• The cache will contain as key - and as value the symbolic name requested
• As DNS packets can contain several responses, the above cache has to contain one entry per response detected
• For each flow, using the above cache, it should be possible to populate the server name even for protocols (e.g. FTP or BitTorrent) that with DPI cannot provide this information