ceph: rewrite msgpool using mempool_t

ceph: rewrite msgpool using mempool_t

Since we don't need to maintain large pools of messages, we can just
use the standard mempool_t.  We maintain a msgpool 'wrapper' because we
need the mempool_t* in the alloc function, and mempool gives us only
pool_data.

Signed-off-by: Sage Weil <sage@newdream.net>