Geo: Fix Dynamic Backoff strategy and add specs

Merged Gabriel Mazetto requested to merge 2639-geo-dynamic-backoff-strategy-is-not-working into master

What does this MR do?

Moves linear_backoff_strategy and geometric_backoff_strategy to class methods in the GeoDynamicBackoff module so a include will work as initially expected.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

In a previous version when we refactored the backoff strategy we actually broke it in the process.

See #2639 (closed)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #2639 (closed)

cc @stanhu @dbalexandre @rspeicher

Edited by Gabriel Mazetto