This removes the transaction-signing code from the hub internal codebase.
The only user for this was the API call signrawtransaction,
and a lot of unit testing code.
I'd argue that if the user needs to send a private key to the hub via
RPC for signing his transaction, then they are doing it wrong.
This also removes the duplicate test double_spend which tests
functionality also tested in the DoubleSpendProofTest.
As an aside, the flowee/utils library still has full signing capability
and we suggest using the TransactionBuilder API for that.
The CAmount name is not helpful as its just an int64_t and not a
class, like the name implies. There were a handful of places where
it was passed in as const-ref, as a good example of this actually
creating sub-par code.
Move the MutableTransactionSignatureChecker class out of the header and
thereby remove the need to include transaction.h in the header.
This checker was never used in any app, just in the unit tests.
This also fixes includes around.