Fixes fast_walker.

The bug was caused by using self._iter_categories, this iterator only returns active (i.e. stumble and walk) candidates, hence no introduced candidates were ever walked too.  Effectively disabling the fast walker.

Improvements include:
- walking to eligible candidates only (i.e. no candidate hammering)
- prematurely abort fast walk when 20 active candidates are available

Depends on https://github.com/Tribler/dispersy/pull/67.

And note that this does not solve the other Candidate/Walker issues that we are seeing.  This problem is still ongoing.