lib: iterator.c: auto-seek: handle new message types

lib: iterator.c: auto-seek: handle new message types

When reading the time of a given message for auto-seeking purposes:

* Skip packet beginning/end messages as packet object beginning/end time
  properties are about to be removed.

* Use a stream activity beginning time if known, otherwise skip the
  message for infinite and unknown times.

* Use a stream activity end time if known, otherwise skip the message
  for an unknown time (a positive infinite time is always greater than
  any requested time).

* Use the beginning time of a discarded events/packets message because
  this beginning time must be greater than or equal to the previous
  message's time for the same stream.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>