Manually implement request backoff/retry

[MattWellie]

Fixes

• the backoff library doesn't work as expected - despite being configured (via the backoff decorator) to use a fibonacci backoff, expanding up to 200 seconds with each additional failure, it's really waiting only a fraction of a second between retries
• instead of continually attempting to get blood from this particular stone, moving on to a manual reimplementation

Proposed Changes

• removes the backoff library as a dependency and in code
• adds an exponential backoff retry, accepting a range of error modes