Add option `--shm-metadata-msg-size`

[dennisklein]

In ipc:// and tcp:// based shmem transport, small queue sizes are not accurate as by default generous kernel buffer sizes are assigned to the underlying unix/tcp sockets. Currently, the user has no tunable at hand to workaround this because the FairMQ shmem transport uses a fixed size, but tiny metadata msg size (40 bytes). By increasing the --shm-metadata-msg-size the user can make sure to exhaust the kernel buffers on the first message, and as a consequence render the effect of the FairMQ queue sizes accurate (which are passed through to ZeroMQ SND/RCVHWM) for very small numbers.