Make relay crypto run on multiple CPU cores

Splitting relay crypto across multiple CPU cores is less essential than maybe it once was, now that we can use aesni, but it can still be critical-path. It's likely to become important again if we shift to something like Salsa20, or some large-block cipher based on it.

For information on the planned architecture, see https://trac.torproject.org/projects/tor/wiki/org/projects/Tor/MultithreadedCrypto