feat: Add new tunable --shm-metadata-msg-size

The shm metadata msg will be right-padded to the given size. This
tunable may be used to saturate the kernel msg buffers more quickly with
the effect that the ZeroMQ message queue size - on which the FairMQ
shmem transport relies upon - behaves more accurately for very small
queue sizes.

This introduces a change for the meta msg format in the multipart case:
old: | MetaHeader 1 | ... | MetaHeader n |
new: | n | MetaHeader 1 | ... | MetaHeader n | padded to fMetadataMsgSize |
where `n` is a `size_t` and contains the number of following meta headers.
Previously, this number was infered from the msg buffer size itself which is
no longer possible due to the potential padding.

Implements #432

fairmq/devices/startMQBenchmark.sh.in

@@ -72,6 +72,7 @@ SAMPLER+=" --msg-size $msgSize"
 SAMPLER+=" --multipart $multipart"
 SAMPLER+=" --num-parts $numParts"
 SAMPLER+=" --shm-throw-bad-alloc false"
+# SAMPLER+=" --shm-metadata-msg-size 1024"
 # SAMPLER+=" --msg-rate 1000"
 SAMPLER+=" --max-iterations $maxIterations"
 SAMPLER+=" --channel-config name=data,type=pair,method=bind,address=tcp://127.0.0.1:5555"