Using the WalkCandidate stored in the request, walkcandidates then to
get lost in Dispersy. Hence, we need to use the one when we attempted to
send the introduction_request.
Merge pull request #289 from NielsZeilemaker/discoveryv1.3

Using the WalkCandidate stored in the request
remove walk_advice* statistics
remove walk_bootstrap* statistics
change some statistics to properties
remove WalkStatistics
fix rebase bug
change for optimize mode
simplify endpoint statistics
some cleanup
some fix for tribler
fix rebase problems
some fix
use dispersy.send_packets() instead of endpoint.send()
some fix