All transactions must declare a list of permissions they utilize. The
chain now checks that the signatures are present to satisfy these
permissions, at least theoretically (only partially tested). As the
transaction is evaluated and applied, the message handlers will check
that the required permissions were declared on the transaction.

Also, define the logic to check that an authority is satisfied (only this
part is tested so far)

TODO: Test that transactions are rejected if they do not bear sufficient
signatures
TODO: Make message handlers check the declared permissions are sufficient,
and reject the transaction if they are not.