When the in-memory running rate is zero, the fallback went through
get_incoming_rate, whose previous-SLE lookup matches the same
posting_datetime and can land on a sibling line of the voucher being
replayed. Replace it with get_previous_sle_of_current_voucher excluding
the current voucher, keeping the get_valuation_rate chain when no
previous entry exists. get_incoming_rate is no longer used in this
module.
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