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.