New polling logic for Tor. #250

Poll for outgoing connections each time the network is enabled, then disable polling once our hidden service descriptor has been published. If our contacts do the same, any contacts that are offline when we poll will connect to us when they come online.

This should reduce bandwidth and battery usage. The idea comes from Ricochet:

https://github.com/ricochet-im/ricochet/issues/68

Fixes #250 (closed).