Guac contract changes

[jtremback]

1. Expiration for newChannel to prevent griefing and toxic waste lying around. Also check for similar issues in other functions.
2. Only allow one channel between each pair of addresses.
3. Salt messages to prevent replays between deployments and anything else.

deadline:11/8/2018

[mpapierski]

@jtremback what about hashlocks? Is it safe to remove them from the ABI?

[mpapierski]

@jtremback What do you think about firing blockchain events at the end of functions in guac in a similiar way what Connectix does?