storage: flush/sync contents to disk on file close

Behavior controlled by configuration (default=off)
It is a trade-off between performance and consistency.

References:
[1] https://github.com/golang/go/issues/20599

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>

pkg/api/config/config.go

@@ -21,6 +21,7 @@ type StorageConfig struct {
 	RootDirectory string
 	GC            bool
 	Dedupe        bool
+	Commit        bool
 	StorageDriver map[string]interface{} `mapstructure:",omitempty"`
 }
 
@@ -90,9 +91,10 @@ type LogConfig struct {
 }
 
 type GlobalStorageConfig struct {
-	RootDirectory string
 	Dedupe        bool
 	GC            bool
+	Commit        bool
+	RootDirectory string
 	StorageDriver map[string]interface{} `mapstructure:",omitempty"`
 	SubPaths      map[string]StorageConfig
 }