Use more force on ctrl-c to kill program

[elonen]

During hashing, client and master alike don't immediately exit. Perhaps it would be best to kill asyncio tasks immediately instead of trying to play nice.

Master doesn't write any files, and peers (re)hash their files anyway, so file corruption isn't really an issue.

[elonen]

Also, _hash_file() proceed in smaller bits instead of the whole chunk as in h = HashFunc().update(mm[p:(p+s)]).result() would probably make everything more responsive.