objstorage: optimize MemObj.NewReadHandle

Better implementation for `MemObj.NewReadHandle` which doesn't
allocate (the allocation shows up in profiles and is a red-herring as
the production path would use a preallocated handle).

objstorage/test_utils.go

@@ -57,7 +57,24 @@ func (f *MemObj) Size() int64 {
 	return int64(f.buf.Len())
 }
 
+type memObjReadHandle MemObj
+
+var _ ReadHandle = (*memObjReadHandle)(nil)
+
+func (h *memObjReadHandle) ReadAt(ctx context.Context, p []byte, off int64) error {
+	return (*MemObj)(h).ReadAt(ctx, p, off)
+}
+
+func (h *memObjReadHandle) Close() error {
+}
+
+func (h *memObjReadHandle) SetupForCompaction() {
+}
+
+func (h *memObjReadHandle) RecordCacheHit(ctx context.Context, offset, size int64) {
+}
+
 // NewReadHandle is part of the Readable interface.
 func (f *MemObj) NewReadHandle(readBeforeSize ReadBeforeSize) ReadHandle {
-	return &NoopReadHandle{readable: f}
+	return (*memObjReadHandle)(f)
 }