Refactor BulkWriter retries

There are a number of issues with the current BulkWriter retry implementation:
- the limit of 15 retries is [hard-coded](https://github.com/googleapis/python-firestore/blob/8ddb859c8178043c0dcd09c85d68ba5d311563bf/google/cloud/firestore_v1/bulk_writer.py#L341), not user configurable
- the BulkWriter-level retries doesn't limit the underlying gapi-clevel retries, resulting in potentially very slow retries
- no max backoff cap
- does not check for idempotency, meaning some operations may be retried when they shouldn't
- uses custom hand-rolled retry logic, instead of relying on the standard api_core

I sugest a full internal refactor of the class, keeping the API the same but cleaning up the internals, and surfacing some extra user configuration