Add "delayed" datastore.

This is a partial extraction from go-ipfs/thirdparty/datastore2.

I have added a test.

delayed/delayed.go

+// Package delayed wraps a datastore allowing to artificially
+// delay all operations.
+package delayed
+
+import (
+	ds "github.com/ipfs/go-datastore"
+	dsq "github.com/ipfs/go-datastore/query"
+	delay "github.com/ipfs/go-ipfs-delay"
+)
+
+// New returns a new delayed datastore.
+func New(ds ds.Datastore, delay delay.D) ds.Datastore {
+	return &delayed{ds: ds, delay: delay}
+}
+
+type delayed struct {
+	ds    ds.Datastore
+	delay delay.D
+}
+
+func (dds *delayed) Put(key ds.Key, value interface{}) (err error) {
+	dds.delay.Wait()
+	return dds.ds.Put(key, value)
+}
+
+func (dds *delayed) Get(key ds.Key) (value interface{}, err error) {
+	dds.delay.Wait()
+	return dds.ds.Get(key)
+}
+
+func (dds *delayed) Has(key ds.Key) (exists bool, err error) {
+	dds.delay.Wait()
+	return dds.ds.Has(key)
+}
+
+func (dds *delayed) Delete(key ds.Key) (err error) {
+	dds.delay.Wait()
+	return dds.ds.Delete(key)
+}
+
+func (dds *delayed) Query(q dsq.Query) (dsq.Results, error) {
+	dds.delay.Wait()
+	return dds.ds.Query(q)
+}
+
+func (dds *delayed) Batch() (ds.Batch, error) {
+	return ds.NewBasicBatch(dds), nil
+}
+
+func (dds *delayed) DiskUsage() (uint64, error) {
+	dds.delay.Wait()
+	return ds.DiskUsage(dds.ds)
+}

delayed/delayed_test.go

+package delayed
+
+import (
+	"testing"
+	"time"
+
+	datastore "github.com/ipfs/go-datastore"
+	delay "github.com/ipfs/go-ipfs-delay"
+)
+
+func TestDelayed(t *testing.T) {
+	d := New(datastore.NewMapDatastore(), delay.Fixed(time.Second))
+	now := time.Now()
+	k := datastore.NewKey("test")
+	err := d.Put(k, "value")
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = d.Get(k)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if time.Since(now) < 2*time.Second {
+		t.Fatal("There should have been a delay of 1 second in put and in get")
+	}
+}

package.json

@@ -24,6 +24,12 @@
       "hash": "QmYBJ8BXPDTMnzLFdv4rS5kbR1fUFASDVDpK7ZbeWMx6hq",
       "name": "go-check",
       "version": "1.0.0"
+    },
+    {
+      "author": "hsanjuan",
+      "hash": "QmRJVNatYJwTAHgdSM1Xef9QVQ1Ch3XHdmcrykjP5Y4soL",
+      "name": "go-ipfs-delay",
+      "version": "0.0.1"
     }
   ],
   "gxVersion": "0.7.0",