Currently, WiredTigerIndex::seekExact converts the KeyString passed in into BSON and then converts it to KeyString again with a discriminator byte. This is related to how we store RecordId at the end of KeyString and use WT's search_near() and then selectively call prev()/next() on the cursor. In the reverse cursor way, we want to append a kExclusiveAfter so that it can land on the exact match.

This is certainly unideal, as we have to do two extra conversions just for appending a discriminator when it's a reverse cursor. Maybe WiredTigerIndex::seekExact should use a different combinations of WT's search_near(), prev() and next() to find the match so that we don't need to append any discriminator byte. Or maybe we just need to add a helper method on KeyString that could easily append a discriminator byte to itself.