Sometimes nodes send many duplicate blocks. This patch disconnects the peer that's running
behind. This shouldn't happen, but it can and does sometimes, so this code caters for that.

This code also becomes more important as the block download algorithm is changed in future,
such as concurrent block downloads from multiple peers, and retries are sent to other peers
when peers seem unresponsive, only to become responsive again later, by which time
they are sending duplicate blocks.

Conflicts:
	src/main.cpp

Conflicts:
	src/main.cpp