Remove interface communication out of blockchain.py
into network.py. network.py handles protocol requests
for headers and chunks. blockchain.py continues to
handle their analysis and verification.
If an interface provides a header chain that doesn't
connect, it is dismissed, as per a previous TODO comment.
This removes a thread and another source of timeouts.
I see no performance issues with this when truncating the
blockchain.
Rename 'result' to 'header' for clarity.
d16714a1db