In theory this is easier on the garbage collector.  Probably could
reuse more of the intermediary buffers by stashing them in the
connection state, but that makes the code kind of messy.  This should
be an improvement.

This should fix sockets getting stuck in CLOSE_WAIT.

The same algorithm as ScrambleSuit is used, except:
 * SipHash-2-4 in OFB mode is used to create the distribution.
 * The system CSPRNG is used when sampling the distribution.

This fixes most of #3, all that remains is generating and sending a
persistent distribution on the server side to the client.

On second thought instead of using log.Panicf(), panic() and do the
logging with recover().  This somewhat centralizes logging in
obfs4proxy, which will be easier to change when I invariably decide to
do logging differently in the future.

This fixes #1.

This adds preliminary support for data padding by adding another layer
of encapsulation inside each AEAD frame containing a type and length.
For now, data is still sent unpadded, but the infrastructure for
supporting it is mostly there.

Additionally, use log.Panic[f]() instead of panic through out the code
so that some panics are logged.

Write timeouts are obnoxious to handle as the frame encoder state
already is updated to cover the entire payload for the Write() call
that timed out.  In theory it is possible to buffer the pending data,
but that causes Write() to voilate the semantics of the interface.

The current timeout value before the server fails the handshake is
15 s.  This may need to be increased for clients over slow links.

Like ScrambleSuit, a random interval between 1x and 5x of additional
data from the peer is read and immediately discarded before closing.

Additionally, obfs4 will close off invalid connections anywhere between
0 and 60 seconds after it determines that the incoming connection will
never complete the handshake successfully.

 * The old and the busted: obfs4-[client,server].
 * The new hotness: obfs4client.
 * Add obfs4.ServerHandshake() that servers need to call after a
   successful return from Accept().  This allows implementations to
   move the handshake into a goroutine or whatever.