osd: add osd_hack_prune_past_intervals

osd: add osd_hack_prune_past_intervals

Last ditch (but dangerous) method of reducing memory usage for
past_intervals, which can help very very unhappy clusters recovery.
A properly implemented version of this is in luminous.  This hacky
version was used successfully to recover multiple jewel-based clusters,
but is still only recommended for use when the OSD is otherwise unable
to recover.

This change is not cherry-picked from master because luminous implements
a more sophisticated version of this that changes the past intervals
representation entirely, but it is too invasive to backport.  This
workaround to prune just on startup should be sufficient for emergencies.

Signed-off-by: Sage Weil <sage@redhat.com>