refac(exch:offline) move offline exchange to its own package

offline.go

-package bitswap
-
-import (
-	"errors"
-
-	context "github.com/jbenet/go-ipfs/Godeps/_workspace/src/code.google.com/p/go.net/context"
-
-	blocks "github.com/jbenet/go-ipfs/blocks"
-	exchange "github.com/jbenet/go-ipfs/exchange"
-	u "github.com/jbenet/go-ipfs/util"
-)
-
-func NewOfflineExchange() exchange.Interface {
-	return &offlineExchange{}
-}
-
-// offlineExchange implements the Exchange interface but doesn't return blocks.
-// For use in offline mode.
-type offlineExchange struct {
-}
-
-// Block returns nil to signal that a block could not be retrieved for the
-// given key.
-// NB: This function may return before the timeout expires.
-func (_ *offlineExchange) Block(context.Context, u.Key) (*blocks.Block, error) {
-	return nil, errors.New("Block unavailable. Operating in offline mode")
-}
-
-// HasBlock always returns nil.
-func (_ *offlineExchange) HasBlock(context.Context, blocks.Block) error {
-	return nil
-}

offline_test.go

-package bitswap
-
-import (
-	"testing"
-
-	context "github.com/jbenet/go-ipfs/Godeps/_workspace/src/code.google.com/p/go.net/context"
-
-	u "github.com/jbenet/go-ipfs/util"
-	testutil "github.com/jbenet/go-ipfs/util/testutil"
-)
-
-func TestBlockReturnsErr(t *testing.T) {
-	off := NewOfflineExchange()
-	_, err := off.Block(context.Background(), u.Key("foo"))
-	if err != nil {
-		return // as desired
-	}
-	t.Fail()
-}
-
-func TestHasBlockReturnsNil(t *testing.T) {
-	off := NewOfflineExchange()
-	block := testutil.NewBlockOrFail(t, "data")
-	err := off.HasBlock(context.Background(), block)
-	if err != nil {
-		t.Fatal("")
-	}
-}