The only SHA-1 hash that remains is used in the permutation of nodeids,
where we need to decide if we care about performance or long-term security.
I suspect that we could use a much weaker hash (and faster) hash for
this purpose. In the long run, we'll be doing thousands of such hashes
for each file uploaded or downloaded (one per known peer).
In various cases, including Merkle Trees, it is useful to tag and encode the inputs to your secure hashes to prevent security flaws due to ambiguous meanings of hash values.
