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
