diag/net: io must respect timeout ctx

See the discussion below. A future commit will implement the closer
change below, and rebase this one on top.

<•jbenet> `n.Diagnostics.GetDiagnostic(time.Second * 20)` is not being respected. should it use a context instead? or is it a timeout because the timeout is sent to other nodes?
<•jbenet> oh it's that the io doesnt respect the context so we're stuck waiting for responses.
<•jbenet> this is that complex interface point between the world of contexts, and the world of io. ctxutil.Reader/Writer is made for this, but you have to make sure to defer close the stream. (see how dht_net uses it). i'd love to find a safer interface. not sure what it is, but we have to a) respect contexts, and b) allow using standard io.Reader/Writers. Maybe TRTTD
<•jbenet> is have ctxutil.Reader/Writer take ReadCloser and WriteClosers and always close them. the user _must_ pass an ioutil. NopCloser to avoid ctxutil closing on you when you dont want it to.
<•jbenet> this seems safer to me in the general case.

diagnostics/diag.go

@@ -16,6 +16,7 @@ import (
 	"github.com/jbenet/go-ipfs/Godeps/_workspace/src/code.google.com/p/go.net/context"
 	ggio "github.com/jbenet/go-ipfs/Godeps/_workspace/src/code.google.com/p/gogoprotobuf/io"
 	"github.com/jbenet/go-ipfs/Godeps/_workspace/src/code.google.com/p/goprotobuf/proto"
+	ctxutil "github.com/jbenet/go-ipfs/util/ctx"
 
 	host "github.com/jbenet/go-ipfs/p2p/host"
 	inet "github.com/jbenet/go-ipfs/p2p/net"
@@ -220,8 +221,10 @@ func (d *Diagnostics) sendRequest(ctx context.Context, p peer.ID, pmes *pb.Messa
 	}
 	defer s.Close()
 
-	r := ggio.NewDelimitedReader(s, inet.MessageSizeMax)
-	w := ggio.NewDelimitedWriter(s)
+	cr := ctxutil.NewReader(ctx, s) // ok to use. we defer close stream in this func
+	cw := ctxutil.NewWriter(ctx, s) // ok to use. we defer close stream in this func
+	r := ggio.NewDelimitedReader(cr, inet.MessageSizeMax)
+	w := ggio.NewDelimitedWriter(cw)
 
 	start := time.Now()