Add support for connecting to I2P network via proxy

[Reportage]

In situations where a direct connection to the internet is unavailable, but
a connection via a proxy is possible, currently I2P will not work. Adding
support for connection to the network via a proxy, leveraging the work done
with reseeding, would allow I2P usage in contexts where it currently isn't
possible.

[zzz]

Not a new idea of course. See e.g. #648 which is similar but different.

Most recently discussed in the context of pluggable transports #1170

The work done in #423 #1130 #1841 helps a little, particularly pulling the SOCKS 5 client code from i2ptunnel into a lib, but the NTCP and SSU transports are not HTTP of course. SSU in particular would be problematic. But it all would affect the IP detection/verification system and other IP checks and uses in the transport subsystem.

Possible first step is to do NTCP only and just disable SSU. But nothing would happen soon.

Would need to hear from actual or potential users on what's needed. Maybe best course is to see if we get more uptake after 0.9.33 is out with the reseed proxy options, then go from there.

ref: ​http://zzz.i2p/topics/2302

[Reportage]

Potential contexts:

• university dorm (great bandwidth!) or other location where all internet access is via a (http) proxy
• users that want/need to hide the fact that they're running I2P locally, and have access to a safe proxy that they can tunnel i2p through (likely useful in countries where i2p use is monitored/prohibited, which may soon include Russia)