OSDMap: add primary out params to _pg_to_osds and _raw_to_up_osds

OSDMap: add primary out params to _pg_to_osds and _raw_to_up_osds

Switch to use pointers for the out parameters instead of references.

These functions are still just pointing at the front of the generated
lists for the "primary" params, but now that all their callers respect
these outputs we can add programmatic leader assignment with just these
two functions.

Signed-off-by: Greg Farnum <greg@inktank.com>