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Instead of threading the code, move the keypair generation to right
after Accept() is called. This should mask the timing differential due
to the rejection sampling with the noise from the variablity in how
long it takes for the server to get around to pulling a connection out
of the backlog, and the time taken for the client to send it's portion
of the handshake.
The downside is that anyone connecting to the obfs4 port does force us
to do a bunch of math, but the obfs4 math is relatively cheap compared
to it's precursors.
Fixes #9.
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Part of issue #9.
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* handhake_ntor_test now is considerably more comprehensive.
* The padding related constants in the spec were clarified.
This breaks wireprotocol compatibility.
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This is done by maintaining a map keyed off the SipHash-2-4 digest of
the MAC_C component of the handshake. Collisions, while possible are
unlikely in the extreme and are thus treated as replays.
In concept this is fairly similar to the ScrambleSuit `replay.py` code,
with a few modifications:
* There is a upper bound on how large the replay filter can grow.
Currently this is set to 102400 entries, though it is unlikely that
this limit will be hit.
* A doubly linked list is also maintained parallel to the map, so the
filter compaction process does not need to iterate over the entire
filter.
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