osd: implement pg merge

osd: implement pg merge

- Vevamps the split tracking infrastructure, and adds new tracking for
upcoming merges in consume_map.  These are now unified into the same
identify_ method.  these consume the new pg_num change tracking
instructure we just added in the prior commit.
- PGs that are about to merge have a new wait infrastructure, since all
sources and the target have to reach the target epoch before the merge
can happen.
- If one of the sources for a merge does not exist, we create an empty
dummy PG to merge with.  The implies that the resulting merged PG will
be incomplete (and mostly useless), but it unifies the code paths.
- The actual merge (PG::merge_from) happens in advance_pg().

Fixes: http://tracker.ceph.com/issues/85
Signed-off-by: Sage Weil <sage@redhat.com>