SERVER-42972 allowed seekExact take a KeyString, but implementations really only search for approximate KeyStrings that match a prefix. This is because most indexes (non-_id) append RecordIds to their keys.

We can narrow the scope of the function so that it's semantics are more obvious. That is: only return exact KeyString matches. Don't try to compare without the RecordId just to satisfy the API.

Additionally, IndexAccessMethod::findSingle() is the only caller of SortedDataInterface::seekExact. And index lookups on the _id index are the only callers of findSingle().

For example: all indexes but the _id index store keys as the concatenation of KeyString+RecordId. If I seek with seekExact on the _id index, I simply pass that key to the storage engine and get a match. If I seek with seekExact on a non-_id index, it would be incorrect to ever return a key, because there will never be an exact match unless a RecordID is also part of the seeked-for KeyString (and that is pointless).