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.