retry: Make content_add endpoints maximize content writes to storage

In effect, allows to write all but the colliding contents to the storage. This is the same behavior currently existing in the journal replayer (swh-journal!124 (closed)). This shares the behavior within the retry proxy as this is a common need, for example in loaders as well (swh-loader-core!379 (closed)).

And the colliding hashes are stored in the log in a formatted way so we do not lose that information.

Related to swh-journal!124 (closed) Related to swh-loader-core!379 (closed)

Note: It could go in an entirely different proxy. That would maximize composition through configuration and not entangle the retry and that new behavior (which is somehow a retry, thus why it's proposed here in the first place ;)

Test Plan

tox

---

Migrated from D3012 (view on Phabricator)

swh/storage/tests/conftest.py

@@ -261,7 +261,7 @@ def sample_data() -> Dict:
     from .storage_data import data
 
     return {
-        "content": [data.cont, data.cont2],
+        "content": [data.cont, data.cont2, data.cont3],
         "content_metadata": [data.cont3],
         "skipped_content": [data.skipped_cont, data.skipped_cont2],
         "person": [data.person],