Rename handshake_digest to rend_circ_nonce

The handshake_digest field was never meaningfully a digest *of* the
handshake, but rather is a digest *from* the handshake that we exapted
to prevent replays of ESTABLISH_INTRO cells.  The ntor handshake will
generate it as more key material rather than taking it from any part
of the circuit handshake reply..