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
Merge remote-tracking branch 'remotes/upstream/devel' into bugfixes
Update statistics from overridden search community.
Requires newer Dispersy.
Merge pull request #187 from boudewijn-tribler/bugfixes

Fixes fast_walker.