Use batch to delete firestore's data in solution branch

[t104360088]

This is the same as PR #31 , but for solution branch

Hi, I found that the way to delete room data in the code is one-by-one deletion, which will make multiple connections to firestore, causing the deletion speed to be slow. Therefore, I use the batch deletion method and submit the batch operation at the end, which is much faster , I hope to contribute to the project to make it better.

The following is the link to the data deleted by firestore in batch： https://firebase.google.com/docs/firestore/manage-data/delete-data?authuser=0#node.js_2