JounralingObjectStore: journal->committed_thru after replay

It's possible that the osd stopped between when the filestore
op_seq file was updated and when the journal was trimmed.  In
that case, it's possible that on boot the journal might be
full, and yet not be trimmed because commit_start assumes
there is no work to do.  Calling committed_thru on the journal
ensures that the journal matches committed_seq.

Fixes: 6756
Signed-off-by: Samuel Just <sam.just@inktank.com>
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>