Objecter: follow redirect replies from the OSD

Objecter: follow redirect replies from the OSD

If we get back a redirect reply, we clean up the Op's external references
and re-send using the target_oloc and target_oid. To facilitate this,
recalc_op_target() now only fills them in and overrides them with pool
cache semantics if they're empty.

Keep in mind that this is a pretty simple redirect formula -- the
Objecter will keep following redirects forever if that's what the OSDs
send back. The client is not providing any synchronization right now.

Signed-off-by: Greg Farnum <greg@inktank.com>