backup target: create snapshot and cache files per target

snapshot file tracks the snapshot directory. when app gets deleted,
the cleaner will remove the upstream snapshot directory when it runs.

cache files are used in rsync logic to track what was uploading into
snapshot in the previous run without needing to rescan upstream.

src/constants.js

@@ -80,6 +80,8 @@ exports = module.exports = {
 
     SECRET_PLACEHOLDER: String.fromCharCode(0x25CF).repeat(8), // also used in dashboard client.js
 
+    SNAPSHOT_INFO_FILENAME: 'snapshot-info.json',
+
     CLOUDRON,
     TEST,