Implement session context

[RafaelAPB]

We have previously stated that clients authorize their gate-
ways to act on their behalf for that specific asset transfer.
This is done leveraging the concept of a session context ctx.
It is calculated by hashing (through a cryptographic hash
function H) the concatenation of the graph D2, with the
current timestamp ts, which avoids replay attacks. This context
is then sent by each Ci to the corresponding Gi along with the
graph D2