[CDAP-772] Addressed some review comments.
[CDAP-772] Made create a POST API since we do not support update yet
[CDAP-772] Re-used app.meta table for namespace metadata
[CDAP-772] Created Id.Namespace and updated NamespaceMetaStore API to use it as key. Moved NamespaceMeta to cdap-proto so cdap-client and cdap-cli could use it later.
[CDAP-772] Reverted create to PUT