prop250: Add commit and SR values generation code

This adds the logic of commit and SR values generation. Furthermore, the
concept of a protocol run is added that is commit is generated at the right
time as well as SR values which are also rotated before a new protocol run.

Signed-off-by: George Kadianakis <desnacked@riseup.net>
Signed-off-by: David Goulet <dgoulet@torproject.org>