writer: make calling flush() in write_addition(s)() optional

using a constructor 'auto_flush' bookean argument. The idea is that in a test session, each call to 'flush()' takes ~1s to run, so having the test handling a single call to flush() when needed (instead of n calls) make test execution significantly faster.

For example, swh-storage's test_replay.py execution went from ~140s to ~40s, and test_backfill.py from ~40s to ~15s.

Depends on !224 (closed)

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Migrated from D8705 (view on Phabricator)