Relaying length-prefixed bins: collect chunks, concatenate once

A relay that does buf = Buffer.concat([buf, chunk]) on every socket chunk while waiting for a binLength-sized payload is quadratic. A 64 MB payload arriving in 4 KB chunks took 13 s on Linux and over 40 s on macOS, because every chunk copies everything held so far. Push the chunks of an incomplete frame (or of a line with no newline yet) into an array with a running length, and call Buffer.concat once when the length reaches what is needed: 64 MB then took 0.15-0.25 s. Test it with a big payload written in small chunks under a time bound plus a sha check, and keep a test that splits headers mid-line and puts the next header in the same chunk as a payload's tail.