osdc/Objecter: avoid locking two sessions at once

... in close_session

Signed-off-by: John Spray <john.spray@redhat.com>
common/shared_cache: take a cct

Signed-off-by: Sage Weil <sage@redhat.com>
common/shared_cache: dump weak refs on shutdown

Before asserting, dump what the refs are along with the counts.

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