Mixnet: Use max_message_size and max_path_length for Sphinx encoding

[youngjoon-lee]

We've defined two parameters: max_message_size and max_mix_path_length, but we haven't used it for Sphinx encoding because the pysphinx library didn't support those parameters.
Now that I modified the pysphinx to accept those parameters (instead of using hard-coded parameters), we can pass those parameters to pysphinx.

As a reminder,

• The size of Sphinx packet is determined by those two parameters. Those are global parameters shared to all nodes in the network.
• All Sphinx packets have the same size, even if an actual message in several packets is shorter than max_message_size.
• All Sphinx packets have the same size, even if an actual mix path of several packets is shorter than max_mix_path_length.
• If an user tries to use values longer than those parameters, pysphinx raises an exception.

[youngjoon-lee]

Closing this with the same reason mentioned in #98 (comment)