datagram: Add support for Datagram2

[altonen]

https://geti2p.net/spec/datagrams#id9

[zzzi2p]

Low priority, won't be needed until UDP tracker proposal http://i2p-projekt.i2p/spec/proposals/160 is updated, reviewed, approved, and implemented, unless some other application designs DG2 in first.

[sjlgasnier]

The UDP tracker proposal was approved a few weeks ago and is being implemented : https://geti2p.net/spec/proposals/160-udp-trackers