libceph: allocate ceph messages with a slab allocator

Create a slab cache to manage ceph_msg structure allocation.

This is part of:
    http://tracker.ceph.com/issues/3926

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: allocate ceph message data with a slab allocator

Create a slab cache to manage ceph_msg_data structure allocation.

This is part of:
    http://tracker.ceph.com/issues/3926

Signed-off-by: Alex Elder <elder@inktank.com>
libceph: use slab cache for osd client requests

Create a slab cache to manage allocation of ceph_osdc_request
structures.

This resolves
    http://tracker.ceph.com/issues/3926

Signed-off-by: Alex Elder <elder@inktank.com>