Add new folder operation, PRELOAD-FOLDER-OUTLINES, which is a hint to

Add new folder operation, PRELOAD-FOLDER-OUTLINES, which is a hint to
the folder implementation that the front end is about to request
header and length information for all messages in the folder.  This
allows the folder to choose a more-optimal download strategy.  The
operation is invoked by the folder-summary code.

Ad-hoc testing indicates that this has a qualitative effect on
performance for folders containing about 100 or more messages.