add initial implementation of remote backing store

This commit implement the BackingStore Interface for remote backing
store.

Signed-off-by: Utkarsh Mani Tripathi <utkarsh.tripathi@mayadata.io>

pkg/api/types.go

@@ -17,6 +17,7 @@ package api
 
 import (
 	"errors"
+	"io"
 	"sync"
 
 	uuid "github.com/satori/go.uuid"
@@ -406,3 +407,14 @@ type UnmapBlockDescriptor struct {
 	Offset uint64
 	TL     uint32
 }
+
+type ReaderWriterAt interface {
+	io.ReaderAt
+	io.WriterAt
+}
+
+type RemoteBackingStore interface {
+	ReaderWriterAt
+	Sync() (int, error)
+	Unmap(int64, int64) (int, error)
+}