mfs write performance

[Benn Snyder]

Some tools like wget and curl write in small chunks of 4096, which is dreadfully slow.
Need to optimize for this common case.

• workaround with wget "${URI}" -O- | mbuffer -s 256k -o "${OUT}"
• fuse-v2 still seems limited to 128K even with -o big_writes
• fuse-v3 has writeback cache, but is not yet supported possibly abandoned upstream
• experimental and still badly broken: mfs-writeback and mfs-writethrough