prevent sending of same block to a peer twice
same block cant be sent twice to a peer within a certain time period
log when dupe block is prevented
Merge pull request #409 from jbenet/no-dupe-send

prevent sending of same block to a peer twice
net/conn: Added bandwidth stats to Conn (BytesRead(), BytesWritten())
core/commands: Added peer bandwidth stats to 'swarm peers'
Vendor the facebookgo/counting package