Fix bugs in search_for_missing, _process_pg_info

Fix bugs in search_for_missing, _process_pg_info

PG::search_for_missing: fix a bug with the handling of MSG_OSD_PG_INFO
messages. Formerly, when processing these messages, we erroneously
assumed that there was nothing missing on the peer at all even in cases
where there were missing objects.

PG::merge_log: drop unused Missing parameter.

_process_pg_info: Don't assume that just because we requested a Log
message at some point, that that is the message we're prcessing.
Correctly handle cases where we didn't get the peer's Missing set or
Log.

Signed-off-by: Colin McCabe <colinm@hq.newdream.net>