Code restructuring, assigning proper subsystems to each

- Drivers contain
    * donut/*
    * file/*
    * memory/*

 - Storage format contains
    * donut/*
      - GetObject() --> renamed to GetObjectReader()
      - Deleted stale objectwriter.go, renamed donutwriter.go to object_writer.go

Simplifying, and documenting codebase further

pkg/storage/donut/object_writer.go

@@ -0,0 +1,88 @@
+package donut
+
+import (
+	"encoding/json"
+	"io/ioutil"
+	"os"
+	"path"
+)
+
+func newDonutObjectWriter(objectDir string) (Writer, error) {
+	dataFile, err := os.OpenFile(path.Join(objectDir, "data"), os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0600)
+	if err != nil {
+		return nil, err
+	}
+	return donutObjectWriter{
+		root:          objectDir,
+		file:          dataFile,
+		metadata:      make(map[string]string),
+		donutMetadata: make(map[string]string),
+	}, nil
+}
+
+type donutObjectWriter struct {
+	root          string
+	file          *os.File
+	metadata      map[string]string
+	donutMetadata map[string]string
+	err           error
+}
+
+func (d donutObjectWriter) Write(data []byte) (int, error) {
+	return d.file.Write(data)
+}
+
+func (d donutObjectWriter) Close() error {
+	if d.err != nil {
+		return d.err
+	}
+	metadata, _ := json.Marshal(d.metadata)
+	ioutil.WriteFile(path.Join(d.root, "metadata.json"), metadata, 0600)
+	donutMetadata, _ := json.Marshal(d.donutMetadata)
+	ioutil.WriteFile(path.Join(d.root, "donutMetadata.json"), donutMetadata, 0600)
+
+	return d.file.Close()
+}
+
+func (d donutObjectWriter) CloseWithError(err error) error {
+	if d.err != nil {
+		d.err = err
+	}
+	return d.Close()
+}
+
+func (d donutObjectWriter) SetMetadata(metadata map[string]string) error {
+	for k := range d.metadata {
+		delete(d.metadata, k)
+	}
+	for k, v := range metadata {
+		d.metadata[k] = v
+	}
+	return nil
+}
+
+func (d donutObjectWriter) GetMetadata() (map[string]string, error) {
+	metadata := make(map[string]string)
+	for k, v := range d.metadata {
+		metadata[k] = v
+	}
+	return metadata, nil
+}
+
+func (d donutObjectWriter) SetDonutMetadata(metadata map[string]string) error {
+	for k := range d.donutMetadata {
+		delete(d.donutMetadata, k)
+	}
+	for k, v := range metadata {
+		d.donutMetadata[k] = v
+	}
+	return nil
+}
+
+func (d donutObjectWriter) GetDonutMetadata() (map[string]string, error) {
+	donutMetadata := make(map[string]string)
+	for k, v := range d.donutMetadata {
+		donutMetadata[k] = v
+	}
+	return donutMetadata, nil
+}