retry: Add retry behavior on pipeline storage with flushing failure

Currently, wrong "hash collisions" are happening a lot on ingestion [1] [2] [3]
The last loading step (flush) is failing on most loaders (git, npm, etc...).

This commits adds the retry behavior to the current pipeline storage deployed.
Which should decrease the frequency of that error.

The remaining hash collision which won't subside should be then real hash
collisions.

[1] https://sentry.softwareheritage.org/share/issue/102aace238fe4ba6b49bcc5531f7c2bf/

[2] https://sentry.softwareheritage.org/share/issue/8e8b48a1d94c465b8109e76311ecdbe7/

[3] https://sentry.softwareheritage.org/share/issue/d4f1208b7eec4b43b11e38494ff039cc/