Previous/back button click does not trigger subset pattern logic

[suryabulusu]

Say we jump to question 32 (bucket 4) from question 1 -- subset pattern logic gets triggered and questions in bucket 4 get fetched.
Now we press back button thrice to arrive at question 29 (bucket 3). Note that this has not been visited yet.
We get a blank question screen -- because bucket 3 hasn't been fetched at all.

We can solve this by adding similar bucket-fetch logic for previous/back button press