Use calculate_max_sqlite_variables to avoid errors in merge_users

[jamalex]

Summary

We were seeing sqlite3.OperationalError: too many SQL variables errors on a tablet running the merge_users command, due to large numbers of bulk-created log records. This PR uses the maximum SQLite variable calculation from Morango to scale back the batch size as needed to avoid the error.

Uncertainty: do we want to cap the max at something lower than what the SQLite variable max might be? And do we want something larger by default for non-SQLite databases?

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[jamalex]

(I used an inline conditional because the linting was complaining about function complexity and I didn't have it in me to try to start refactoring other stuff :) )

[jredrejo]

LGTM!