optionally re-enable start topology

[lpirl]

Sorry for annoying you folks with my use case again:

As a consequence of fixing #2759 (with ​this commit), Tahoe cannot be operated in a star topology anymore (as also stated in ​the change log).

IMHO, this is a very valid use case, since NAT is still reality and we do not have any hole-punching etc. yet. Using an overlay network is not always an option (resource constraints, constraints regarding the administrative rights, etc.).

Looking at the diff of the commit linked above, I am not enough into the code to see if/how we could easily implement a switch for either using a separate Tub per connection or not.

Could a switch to re-enable reverse connections to storage servers be implemented "easily" (undoing the privacy improvement for the sake of a working deployment)?

For the environment that I have, I cannot use Tahoe>=1.12.0 at all (well, #2759 is still a privacy improvement but the consequences feel a bit too drastic :)).

[exarkun]

This ticket should probably be, instead, something like "Support storage servers behind NAT". Optionally re-enabling the tub re-use that was removed in the linked commit might be one way to address that. But another might be STUN or TURN or UPnP or hole punching.