Allow scoped_exit objects to be canceled so they do not call their
callbacks when destroyed.

Move logic from apply_context::unused_authorizations out to a header so
other code can use it.

The logic in question takes an array of data, and a parallel array of
"markers" (booleans, in this instance), and returns a range containing
only the data elements whose corresponding marker matches a provided
value (i.e. the boolean was true).

Stay tuned to see how else I use this code! ;)

I want two things from AuthorityChecker that it wasn't doing yet:
1. I want it to track which of the provided keys were used, so I can
reject transactions which bear more signatures than are necessary
2. To sign a transaction with no unnecessary keys, I want it to support
taking a list of available public keys and an authority, then telling me
which of those keys I should use to fully satisfy the authority, without
having any unnecessary keys

As an added bonus, having both of these operations handled by
AuthorityChecker means that determining the set of keys needed to sign a
transaction, and determining whether a transaction is properly signed,
use the same code. :D

Implement automatic signing of transactions in the test framework.
Currently disabled, as it's not yet working

Previously, validate_referenced_accounts was called during transaction
application; however, account creation is one of those changes that
doesn't really take effect until the end of the block, so we want to
validate the referenced accounts for all transactions at the beginning
of the block so that if trx A creates account joe, then trx B references
joe, that reference will fail if A and B are in the same block. We also
call it during push_transaction, as we do want to check it for pending
transactions that aren't in a block yet.