NetFlow version 1, 5, 7, 8, 9 & 10 (IPFIX) support for Go

We're still in a draft status, do not use this for processing NetFlows in an untrusted environment.

• Expire templates with empty fields (v9)
• Make IPFIX usable (v10)
• Define a more clear interface for caching

Please submit your pull requests and/or bug reports via GitHub.