Add a new service to allow Anonymous Users to vote on xblock-poll

[Danyal-Faheem]

Changes

• Service AnonymousUserPollService was added to the edx-platform runtime.
• This service was needed to allow anonymous users to vote on polls using xblock-poll as the FieldDataCache object was not caching data for anonymous users and the ORM would try to create new entries in the table with the same primary key, thereby resulting in an integrity error.
• The service has a method get_tally which just gets the tally of the poll from the database.
• The service has another method vote which updates the tally in the database based on the choice of the anonymous user.