encoding/gob: reduce decoder memory

encoding/gob: reduce decoder memory
Gob decoding reads a whole message into memory and then
copies it into a bytes.Buffer. For large messages this wastes
an entire copy of the message. In this CL, we use a staging
buffer to avoid the large temporary.

Update #2539
RSS drops to 775MB from 1GB.
Active memory drops to 858317048 from 1027878136,
essentially the size of one copy of the input file.

R=dsymonds, nigeltao
CC=golang-dev
https://golang.org/cl/6392057