osd: set STRAY on pg load when non-primary

osd: set STRAY on pg load when non-primary

The STRAY bit indicates that we should annouce ourselves to the primary,
but it is only set in start_peering_interval().  We also need to set it
initially, so that a PG that is loaded but whose role does not change
(e.g., the stray replica stays a stray) will notify the primary.

Observed:
 - osd starts up
 - mapping does not change, STRAY not set
 - does not announce to primary
 - primary does not re-check must_have_unfound, objects appear unfound

Fix this by initializing STRAY when pg is loaded or created whenever we
are not the primary.

Fixes: #2866
Signed-off-by: Sage Weil <sage@inktank.com>