implement mark and sweep GC

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

dont GC blocks used by pinner

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

comment GC algo

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

add lock to blockstore to prevent GC from eating wanted blocks

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

improve FetchGraph

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

separate interfaces for blockstore and GCBlockstore

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

reintroduce indirect pinning, add enumerateChildren dag method

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

merkledag_test.go

@@ -164,7 +164,7 @@ func runBatchFetchTest(t *testing.T, read io.Reader) {
 
 	spl := chunk.NewSizeSplitter(read, 512)
 
-	root, err := imp.BuildDagFromReader(dagservs[0], spl, nil)
+	root, err := imp.BuildDagFromReader(dagservs[0], spl)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -306,7 +306,7 @@ func TestFetchGraph(t *testing.T) {
 	}
 
 	read := io.LimitReader(u.NewTimeSeededRand(), 1024*32)
-	root, err := imp.BuildDagFromReader(dservs[0], chunk.NewSizeSplitter(read, 512), nil)
+	root, err := imp.BuildDagFromReader(dservs[0], chunk.NewSizeSplitter(read, 512))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -333,7 +333,7 @@ func TestEnumerateChildren(t *testing.T) {
 	ds := NewDAGService(bsi[0])
 
 	read := io.LimitReader(u.NewTimeSeededRand(), 1024*1024)
-	root, err := imp.BuildDagFromReader(ds, chunk.NewSizeSplitter(read, 512), nil)
+	root, err := imp.BuildDagFromReader(ds, chunk.NewSizeSplitter(read, 512))
 	if err != nil {
 		t.Fatal(err)
 	}