Fix statesman index detection
Fix statesman index detection

The previous logic worked fine with parent tables that began with the
letters A-S, but with tables that started with the letters T-Z, the
comparison would fail, which ultimately resulted in the RecordNotUnique
exception not being converted to a TransitionConflictError. Among other
things, this prevents `retry_conflicts` from working.

gocardless/statesman,gocardless/statesman,timothyp/statesman,timothyp/statesman