BLK-87 Simplifies duplicate post logic and adds a test

Signed-off-by: Oren Leiman <oleiman@neutral.io>
BLK-87 If prev_freshest is None, RESTPolling.find_fresh_posts will compare post epoch to 0, i.e. include the post by default

Signed-off-by: Oren Leiman <oleiman@neutral.io>
Merge pull request #8 from nio-blocks/feature/simplify_rest

Feature/simplify rest