gnlcomposition: Don't flush_start / flush_stop according to user's flags.

We do it ourselves when needed, doing so led the pipeline to lose state,
which was updating the base time.
fixes https://bugzilla.gnome.org/show_bug.cgi?id=701146
gnlcomposition: forward non time-resetting flush stops outside the composition,
except those consecutive to a seek event.

The 'reset-time' parameter has been added in 1.0, and we need it to be FALSE
outside the composition.

Related to https://bugzilla.gnome.org/show_bug.cgi?id=701146
composition: update operations base time even when we don't update the pipeline.