Previously, a timeout (in int64 nanoseconds) applied to a granularity
even smaller than one operation:  a 100 byte read with a 1 second timeout
could take 100 seconds, if the bytes all arrived on the network 1 second
apart.  This was confusing.

Rather than making the timeout granularity be per-Read/Write,
this CL makes callers set an absolute deadline (in time.Time)
after which operations will fail.  This makes it possible to
set deadlines at higher levels, without knowing exactly how
many read/write operations will happen in e.g. reading an HTTP
request.

Fixes #2723

R=r, rsc, dave
CC=golang-dev
https://golang.org/cl/5555048