msg/async/Stack: add abstract Stack

msg/async/Stack: add abstract Stack

Stack is a network IO framework which encapsulates all necessary basic network
interface, then it manages threads to work.

Different network backend like posix, dpdk even RDMA need to inherit Stack
class to implement necessary interfaces. So it will make ease for other
network backend to integrated into ceph. Otherwise, each backend need to
implement the whole Messenger logics like reconnect, policy handle, session
maintain...

Signed-off-by: Haomai Wang <haomai@xsky.com>