This required some refactoring when handling encrypted payloads.
Also changed log messages so that "encrypted payload" is logged instead
of "encryption payload" (even if we internally still call it that) as
that's the name used in RFC 5996.
Its functionality can be replicated by calling insert_before() followed
by remove_at(). Not the other way around, though, because remove_at()
changes the enumerator position.
Tobias Brunner