Use callbacks in SlruScanDirectory for the actual action

Previously, the code assumed that the only possible action to take was
to delete files behind a certain cutoff point.  The async notify code
was already a crock: it used a different "pagePrecedes" function for
truncation than for regular operation.  By allowing it to pass a
callback to SlruScanDirectory it can do cleanly exactly what it needs to
do.

The clog.c code also had its own use for SlruScanDirectory, which is
made a bit simpler with this.