We now check authorization of transactions when they come in as pending,
and again when we apply them in some block (either when generating the
block or when applying it). When applying the transactions in a block,
we check auth for all transactions in the block prior to processing any.

To check auth, we first scan all of the declared authorizations, and
check that the authorization is sufficient to grant permission for the
given account to execute the given message type (TODO: look up the
actual required permission level rather than just assuming it's
'active'); then, check that the transaction bears signatures to confer
the declared authorization.