hs: Change default HiddenServiceVersion to 3
The next tor release (0.3.5) is an LTS, it would mean that keeping v2 the default version forces us to support it until 2020 (minimum).
Hidden service version 3 have been merged back in 0.3.2 and have been extensively tested. Bugs remains but we are confident they can be addressed in 0.3.5 and aren't blockers to the mass adoption of v3.
By making version 3 the default version for new services, we can start working on deprecating version 2 in future tor releases.
Still, for a while, v2 will work on the network so this is not about removing v2 entirely but simply switching new default services to v3. An operator will still be able to specify HiddenServiceVersion 2 explicitly if they do really want a v2.
NOTE: There is a tricky issue with this. We'll have to make tor "guess" the right version when loading the configuration. Right now, we explicitly ask the operator to provide HiddenServiceVersion 3 else we consider it a v2.
Considering that, we can't ask all operators to add HiddenServiceVersion 2 to their torrc so we need to make tor smart so it can learn the right version if not given.