mgr: rework kv store load path

mgr: rework kv store load path

The locking and blocking around this was a bit
tricky.  Do the simple thing, and pull the
load_store out to Mgr so that it can be safely
done as part of the background_init process (just drop
Mgr::lock across blocking actions).

Signed-off-by: John Spray <john.spray@redhat.com>