Change clear collections to be a batch function

- Firebase only supports 500 changes as part of a batch, so this
  function needs to loop through larger collections.

lib/databases/firestore.js

@@ -41,18 +41,34 @@ function firestoreQueryParser(collectionRef, queryParams) {
 };
 
 function emptyCollection(db, collection, callback) {
+  var batchSize = 300;
   var collectionRef = db.collection(collection);
-  var query = collectionRef.get().then(function (querySnapshot) {
-    var writeBatch = db.batch();
-    querySnapshot.forEach(function (documentSnapshot) {
-      var documentPath = collection + '/' + documentSnapshot.id;
-      var documentRef = db.doc(documentPath);
-      writeBatch.delete(documentRef);
-    });
-    writeBatch.commit().then(function () {
-      if (callback) callback(null);
-    });
-  });
+  var query = collectionRef.limit(batchSize);
+
+  return deleteQueryBatch(db, query, batchSize, callback);
+}
+
+function deleteQueryBatch(db, query, batchSize, callback) {
+  query.get()
+    .then((snapshot) => {
+      if (snapshot.size == 0) {
+        return 0;
+      }
+
+      var batch = db.batch();
+      snapshot.docs.forEach((doc) => batch.delete(doc.ref));
+      return batch.commit().then(() => {
+        return snapshot.size;
+      });
+    }).then((numDeleted) => {
+      if (numDeleted == 0) {
+        return callback();
+      }
+
+      process.nextTick(() => {
+        deleteQueryBatch(db, query, batchSize, callback);
+      });
+    }).catch(callback);
 };
 
 function getPrecondition(vm) {