implement "new" qemu feature discard-zeroes

[vtolstov]

Sometime ago qemu have detection of writing zeroes and transparently can du unmap requst to underlining storage.
I think for sheepdog it easy can be implemented to use OP_DISCARD with offset and len.
In this case then user try to write file with zeroes and file size more then one block we can fallocate it to zero hole.
If written data less then block - we can split request to discard/write new or do like now - detect zeroes on sheep side.
What do you think?