Issue 15:	Stress test corrupts volume

Checked in for GThomas(tgulacsi78)

I've also met with partial writes (pipe closed on localhost), and also
met with real corruption: after some partial write, the data size in
needle header was read as some huuuuge number, which resulted in memory
panic.

Please consider the attached patch for ensuring full writes (seeks back
to the beginning on needle append error). Hope this helps.
Tested with a small tmpfs, lot of "no space left on device" :)
(sudo umount -lf /tmp/weed; mkdir -p /tmp/weed && sudo mount -o
size=128M,mode=4777 -t tmpfs tmpfs /tmp/weed && bin/weed master
-mdir=/tmp/weed -debug=true & bin/weed volume -dir=/tmp/weed
-debug=true)

GThomas