gob: use bufio on the decode to avoid a system call on each read.

gob: use bufio on the decode to avoid a system call on each read.
Add a benchmark.
BenchmarkEndToEndPipe gives 14.3microseconds/op before,
13.1microseconds/op after, or about 76e3 round trips per second
through the kernel.
With a bytes buffer, and therefore no system calls for I/O, the
numbers go to 7.3microseconds/op, or about 137e3 round trips
per second.

R=rsc
CC=golang-dev
https://golang.org/cl/4279045