Flesh out alert blocks

[kpcyrd]

Alert blocks exist in the current implementation, but are currently not fully implemented.

The purpose of an alert blocks is:

• include bytes of the event
• act as an rekey block which destroys the old session signing key
• mark the block as incident

Currently, every info block triggers a second rekey block, since there's no other way to periodically rekey without explicitly triggering rekeys with tr1pctl rekey. I think there are some usecases where not every info block should rekey for performance reasons, but info blocks should be able to act as rekey block as well.

Alert blocks would be more useful if there's a framework that allows triggering actions with alert blocks, so certain events trigger the execution of a script. For sandboxing reasons, this command needs to be executed from a separate process. This could be combined with #18.