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 +41,48 @@ function firestoreQueryParser(collectionRef, queryParams) {
 };
 
 function emptyCollection(db, collection, callback) {
+  console.log('Emptying firestore collection', collection);
+  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);
+
+  console.log('Calling deleteQueryBatch for collection');
+  return deleteQueryBatch(db, query, batchSize, callback);
+}
+
+function deleteQueryBatch(db, query, batchSize, callback) {
+  console.log('In deleteQueryBatch. Get query results');
+  query.get()
+    .then((snapshot) => {
+      console.log('Snapshot returned');
+      console.log('Size', snapshot.size);
+      if (snapshot.size == 0) {
+        console.log('Resolving because size is 0');
+        return new Promise((resolve) => resolve(0));
+      }
+
+      console.log('Setting up batch for deletion');
+      var batch = db.batch();
+      console.log('Setting up docs for deletion');
+      snapshot.docs.forEach((doc) => batch.delete(doc.ref));
+      console.log('Starting deletion commit');
+      return batch.commit().then(() => {
+        console.log('Batch committed. Returning size', snapshot.size);
+        return snapshot.size;
+      }).then((numDeleted) => {
+        console.log('Num deleted', numDeleted);
+        if (numDeleted == 0) {
+          console.log('Call callback as numDeleted == 0');
+          return callback();
+        }
+
+        console.log('Schedule next batch for deletion on next tick');
+        process.nextTick(() => {
+          console.log('TICK! Execute next batch');
+          deleteQueryBatch(db, query, batchSize, callback);
+        });
+      }).catch((e) => {console.error('Error on get', e); callback(e)});
     });
-  });
 };
 
 function getPrecondition(vm) {
@@ -263,14 +293,18 @@ _.extend(Firestore.prototype, {
   },
 
   clear: function (callback) {
+    console.log('Entered clear');
     this.checkConnection();
 
     var self = this;
     if (!this.collection) {
+      console.log('No collection set, just callback');
       if (callback) callback(null);
       return;
     }
 
+    console.log('Calling emptyCollection for', this.collection);
+
     emptyCollection(this.db, this.collection, callback);
   },