Allow access to session_id

[rustaceanrob]

Each packet handler should be one-to-one with a session ID, so I allow access to a field on the struct here. Resolves #61

cc @nyonson @Kixunil

[nyonson]

ACK d3eaa82

I am a little curious what the use case is for this, just to better understand. Like, maybe two parties have multiple encrypted channels between each other?

[Kixunil]

@nyonson detection of active tampering if you can compare the session ID out-of-band.

[Kixunil]

ACK d3eaa82