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>

examples/config-commit.json

@@ -0,0 +1,15 @@
+{
+    "version": "0.1.0-dev",
+    "storage": {
+        "rootDirectory": "/tmp/zot",
+        "commit": true
+    },
+    "http": {
+        "address": "127.0.0.1",
+        "port": "8080",
+        "ReadOnly": false
+    },
+    "log": {
+        "level": "debug"
+    }
+}