Listen after DB Connection is established

[index.js](https://github.com/xceed-nitj/xceed-backend/blob/06b8e0171bda17069f386656405143d1c176afa2/src/index.js#L24)
The server should start listening to requests only after a connection to the database is established. [Code for reference](https://github.com/wdmc-nitj/TeamD-Backend/blob/92622543ddc68125203d5f414e3b3830dbd2e3b4/server.js#L23)