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.
tomFlowee