Copy instead of append small messages into mux segments

An idea from the investigation into https://github.com/input-output-hk/cardano-node/issues/488 .

On the egress side I experimented with copying small messages into the existing mux sdu, similar to what ByteString builder does but with a smaller limit for where copy instead of appending chunks are acceptable. I couldn’t measure any noticeable performance improvement, but it is probably worth revisiting in the future.
The chainsync SDUs are about 600 bytes split into 300 two byte chunks, which can’t be good.