This fixes segfaults as well as corrupt listes when copy/pasting
due to invalid iterators.
::mark_dirty() must be called with WriterLock, and
::rt_safe_earliest_event_linear_unlocked() must not be called
while _events is being modified. The Sequence iterator
(only user of that function) does not ensure this. Only the
sequence read-lock is taken.
These are longer be used since Seuqnce has a "force_discrete"
boolen that needs to be taken into account in addition to
user-configurable ControlList _interpolation mode.
