ceph_context: use condition variable for wake-up

reopen_logs is called from normal thread context (as opposed to an async
signal handler), so we grab a mutex and condition variables. The mutex
use also allows us to avoid volatile variables since the mutex will add
any necessary barriers.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge pull request #771 from ceph/wip-ceph-context

ceph_context: replace semaphore

Reviewed-by: Sage Weil <sage@inktank.com>