Purge the batch cache when unloading a community
Merge pull request #278 from whirm/twisted-callback

Purge the batch cache when unloading a community
I forgot to replace these yields
Merge pull request #280 from whirm/twisted-callback

I forgot to replace these yields
remove walk_advice* statistics
remove walk_bootstrap* statistics
change some statistics to properties
refactory walk statistics
add total_candidates_discovered into CommunityStatistics
small changes
add MessageStatistics
replace success dict and success_count
replace outgoing dict
replace created dict and created_count
small fixes
replace drop dict and drop_count
fix community initialization problem