fix: replace default retry for upload operations

[cojenco]

This PR replaces upload operations default retries with storage.retry.DEFAULT_RETRY_IF_GENERATION_SPECIFIED and aligns with the retry strategy conditional idempotency.

With default retries set to storage.retry.DEFAULT_RETRY_IF_GENERATION_SPECIFIED, passing in ifGenerationMatch makes the upload operation (1) conditional and (2) only retried by default on whether the object's current generation matches the given value. Setting if_generation_match=0 makes the upload operation succeed only if there are no live versions of the object (fresh uploads) and supports retries.

• revises upload operations both in entry methods and private methods
• updates tests
• revises docstrings
• (update: separating this to feat: add preconditions and retry configuration to blob.create_resumable_upload_session #484 ) adds preconditions and retry configuration to an upload entry method blob.create_resumable_upload_session()

Fixes #477 🦕

[andrewsg]

Good work here. Please amend the PR title to include that create_resumable_upload_session is being amended in the same PR (or split it off into a separate PR) so that the change is included in the release notes.

[andrewsg]

Oh, it looks like we're evaluating it BOTH in blob._do_upload() and in client.download_blob_to_file(). This is harmless, which is why the tests didn't catch it, but we only need one of those.

I don't have a strong opinion on whether we do this in _do_upload or download_blob_to_file. Let's keep this docstring change and additionally we can remove the conditional retry policy code in client.download_blob_to_file, either in another PR, or in this one if you prefer.

[cojenco]

IIUC, I believe we'll need to remain both ConditionalRetryPolicy evaluations as one is for upload operations and the other is for downloads. I revised the docstrings here to match particularly where we're doing the evaluations for upload operations, blob._do_upload() .

[tritone]

Nice catch on this! A couple questions