Similar to the previous commit; make sure that the ValidationContext has
access to the new Tx format so new code can use it directly.
In practically all cases (outside of the unit tests) callers already had
a Tx instance. Making it just a matter of sending it with.
Notice that the Tx object is immutable and implicitly shared which makes
it cheaper to pass around.
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