ENG-10472: Update SPI repair log truncation handle when MP commmits.
ENG-10472: Update SPI repair log truncation handle when MP commmits.

Instead of updating the truncation handle when fragments are completed,
update it when the MP complete message is received, which indicates that
the MP transaction is done. This way, the replica's repair log is
guaranteed to only see new truncation handle from the SPI on transaction
boundaries.

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