add a content replayer service

• service that listen to the 'swh.journal.objects.content' topic and replicate object blobs from a source object storage to a destination one.

• refactor replayer tests; implement the whole kafka mocking logic in the Mocked* classes and delete the test_write_replay_same_order which was not testing anything more than what the orther test do.

• client: simplify a bit the JournalClient class no need for those poll() and commit() proxy methods.

Depends on !32 (closed)

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Migrated from D1541 (view on Phabricator)