CLDSRV-776: Fix batchDelete route to handle raftSessionId parameter

The batchDelete route in routeBackbeat.js was not updated when Arsenal
added the raftSessionId parameter to metadata.getBucket() callbacks in
commit 184d37bdd (ARSN-533).

Fixed by adding an intermediate callback that explicitly handles all
3 parameters but only passes the needed 2 to next()

Author: dvasilas

lib/routes/routeBackbeat.js

@@ -1316,7 +1316,8 @@ function batchDelete(request, response, userInfo, log, callback) {
             }
 
             return async.waterfall([
-                next => metadata.getBucket(bucket, log, next),
+                // eslint-disable-next-line no-unused-vars
+                next => metadata.getBucket(bucket, log, (err, bucketMD, raftSessionId) => next(err, bucketMD)),
                 (bucketMD, next) => quotaUtils.validateQuotas(request, bucketMD, request.accountQuotas,
                     ['objectDelete'], 'objectDelete', -contentLength, false, log, next),
             ], err => {