Stop sending the current time in client NETINFO handshakes.

Implements part of proposal 222.
Get ready to stop sending timestamps in INTRODUCE cells

For now, round down to the nearest 10 minutes.  Later, eliminate entirely by
setting a consensus parameter.

(This rounding is safe because, in 0.2.2, where the timestamp mattered,
REND_REPLAY_TIME_INTERVAL was a nice generous 60 minutes.)
Remove the timestamp from AUTHENTICATE cells; replace with random bytes

This isn't actually much of an issue, since only relays send
AUTHENTICATE cells, but while we're removing timestamps, we might as
well do this too.

Part of proposal 222.  I didn't take the approach in the proposal of
using a time-based HMAC, since that was a bad-prng-mitigation hack
from SSL3, and in real life, if you don't have a good RNG, you're
hopeless as a Tor server.
Round down hidden service descriptor publication times to nearest hour

Implements part of proposal 222.  We can do this safely, since
REND_CACHE_MAX_SKEW is 24 hours.
Avoid error by not saying which intro cell type I mean
Merge branch 'no_client_timestamps_024_v2' into maint-0.2.4