Allow more than one peaks_ready callback to be pending for an AudioRegionView

Allow more than one peaks_ready callback to be pending for an AudioRegionView
at any one time.  Prevents problems when there is more than 1 channel for which
peaks are not ready; before, the first peaks_ready callback would be forgotten
when the second one was requested.  Should fix #3074.

git-svn-id: svn://localhost/ardour2/branches/3.0@6951 d708f5d6-7413-0410-9779-e7cbd77b26cf