rename flat-file to rsync

not a name I like but cannot come up with anything better

https://en.wikipedia.org/wiki/Flat_file_database

the term 'rsync format' seems to be used in a few places

src/storage/s3.js

@@ -43,7 +43,7 @@ function mockRestore() {
     AWS = originalAWS;
 }
 
-// TODO: If we decide to use flat-file backups for CaaS, we should cache the credentials below
+// TODO: If we decide to use rsync backups for CaaS, we should cache the credentials below
 function getCaasCredentials(apiConfig, callback) {
     assert.strictEqual(typeof apiConfig, 'object');
     assert.strictEqual(typeof callback, 'function');