Various fixes

[qstokkink]

Fixes #21

This PR:

• Fixes a crash when linking the core to the GUI when requests are already destructed.
• Updates the default minimum circuits to be 6 instead of 1. This should hopefully improve out-of-the-box anon download speeds.

[egbertbouman]

With circuits now being able to do 12+MB/s, I don't think you need 6 circuits to achieve decent speeds. Considering exit nodes allow only a limited amount of circuits, I think we should consider a default of 3, like we do on the main repo.

[qstokkink]

@egbertbouman 👍 fixed.

[egbertbouman]

Thanks!