Abstracted Storage Mechanism and Updated PollsManager

[Gaurav1924]

-Created Polly.StorageBehaviour behaviour to abstract storage operations.

• Implemented Polly.ETSStorage to handle ETS-based storage.
• Updated Polly.PollsManager to use the abstracted storage behaviour.
• Ran the test suite for polls_manager_test

Fixes #8

[girishramnani]

Add tests