input_nmsg: split read_sock() functionality into _input_nmsg_read_container_sock() and new function deserialize_header()
input_nmsg: split read_sock() functionality into _input_nmsg_read_container_sock() and new function deserialize_header()

    this commit also removes support for reading NMSG v1 containers from
    sock stream inputs. the last nmsg version that did not support v2
    containers was 0.0.18, from january 2009.

hstern/nmsg,hstern/nmsg,hstern/nmsg,hstern/nmsg,hstern/nmsg