Doc
Implement pruning

Pruning makes nodes ask downstream neighbours whether the connection can
be dropped without making it cross its minimum neighbour threshold.
Minimizes the number of connections in the network.
Remove DSN from DB when client terminates
Move TODOs to Github issues
Add missing 'show' for Prune signal
Make connections utility take program name as arg
Redundant \n
Mention pruning in readme
Merge branch 'develop'