If in.Mutex is never locked by Handshake when c.handshakeComplete is
true, and since c.handshakeComplete is unset and then set back by
handleRenegotiation all under both in.Mutex and handshakeMutex, we can
significantly simplify the locking strategy by removing the sync.Cond.

See also https://groups.google.com/forum/#!topic/golang-dev/Xxiai-R_jH0
and a more complete analysis at https://go-review.googlesource.com/c/go/+/33776#message-223a3ccc819f7015cc773d214c65bad70de5dfd7

Change-Id: I6052695ece9aff9e3112c2fb176596fde8aa9cb2
Reviewed-on: https://go-review.googlesource.com/33776Reviewed-by: Adam Langley <agl@golang.org>