Support 'cancel or fail' semantics in OfferCreate (RIPD-562):

Currently, if you specify a cancel sequence in OfferCreate, the
offer may or may not be canceled.

This makes it difficult to implement client-facing interfaces
that implement "modify existing order" functionality which are
implemented by doing a cancel for the old order along with a
create with the new parameters.

The new `tfMustCancel` flag that can be specified in OfferCreate
can be used to ensure that the new offer will only be placed if
the cancel request is successful.