indexing-lister: Allow to define flush packet size

Prior to this commit, indexing lister instances were flushing every packet of 20. This can now be defined per sub classes.

For the bitbucket lister, as the number of repositories grew from 10 per page to 100 per page, that enlarged the time frame between flushes.

Depends on !378 (closed)

Test Plan

tox

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Migrated from D1635 (view on Phabricator)