ReplicatedPG: test for missing head before find_object_context

find_object_context doesn't return EAGAIN for a missing head.
I chose not to change that behavior since it might hide bugs
in the future.  All other callers check for missing on head
before calling into find_object_context because we potentially
need head or snapdir to map a snapid onto a clone.

Fixes: 6758
Signed-off-by: Samuel Just <sam.just@inktank.com>