As of `15b960d55905877a840fe605a41a8139bffb5329` goptlib supports
IsClient, IsServer, and handling the StateLocation.

Yes this means you need to use goptlib out of git.

This breaks wireprotocol compatibility.

This fixes #6.

Part of #6, still need to make logs nicer.

 * Fixed where the code wasn't ensuring that the MAC_[C,S] was present.
 * Optimized the server side to only look at the tail of the (possibly
   incomplete handshakeRequest).

This makes it consistent across all incoming connections, for real this
time (oops).

It will vary per bridge as it is based off the DRBG, but ever attempt
at poking at any given bridge will exhibit consistent behavior.

This is more common than 15 seconds (It's what Firefox uses for the
request timeout).

This fixes #4.

Clients will now always add 87 bytes of padding to the clientRequest,
and Servers will always send the PRNG seed frame unpadded, and bundled
with the serverResponse.

Why 87 bytes?  The amount of data that the server sends is 87.

This fixes #5.

Instead of including the previous secretbox in the input when
calculating the  SipHash-2-4 digest used to generate the obfuscation
mask, use only the nonce.  This is significantly faster, and if someone
breaks obfs4 by exploiting the low amount of input entropy between each
invocation (a counter incrementing by 1), I hope they publish the
attack on the PRF.

This breaks wire protocol compatibility.

 * HMAC-SHA256 -> HMAC-SHA256-128.
 * Mark/MAC are now both caluclated using Public Key | NodeID.

This breaks wire protocol compatibility.

This fixes #3, and brings the code to be on par with the delopyed
versions of ScrambleSuit in terms of features.

This also adds the drgb-seed option to the `-gen` obfs4proxy output.

This paves the way for having servers use the same seed for all
incoming connections, across multiple startup/shutdown cycles.  As
opposed to the current situation where each Obfs4Listener will
randomly generate it's seed at creation time.

Additionally, use 256 bit seeds (128 bit SipHash-2-4 key + 16 bytes of
initial material).

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.