Code cleanups

 * Remove a bunch of gotos to avoid being like Apple.
 * Be slightly more proactive about getting rid of the ScrambleSuit
   uniformdh_handshake_ object.
 * Remove the shared_secret_ member object from UniformDHHandshake since it is
   responsible for keying Client and there is no reason to retain it.

I was about to add -Wunreachable-code, but the option doesn't exist anymore in
GCC, and the only place that triggers the warning is in easylogging++.h where
they break; after a return in a switch statement for consistency (checked with
clang++).