osd/ReplicatedPG: make find_object_context() pass missing_oid

osd/ReplicatedPG: make find_object_context() pass missing_oid

Prevoiusly we would return a snapid that we are blocked on if it is
missing.  This is necessary because the missing clone does not always
match the logical snap we are trying to read.

Extend this to return a full hobject_t that is the missing object we want.
For the missing clone case, this cleans things up slightly.  More
importantly, it lets find_object_context also tell us which on-disk
object is missing that, if it could be promoted, would help.

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