OSDMap: implement pg_to_up_acting_osds with primary interface

OSDMap: implement pg_to_up_acting_osds with primary interface

Use our pointer calling conventions instead of a reference for the
new version of the function.

Right now we're just setting the primaries equal to the first member
of up and acting (or -1 if none), but very shortly we'll modify our
private OSDMap functions to export them based on the contents of temp_primary.
While in general anybody querying for the mapping information will
need to pay attention to whom the primary is as well, we have lots
of callers who will need real code changes to do so. To serve them,
we keep a version that does not export the primary, but asserts
that the primary matches the first entry in its list.

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