Re: MarkBufferDirty Assert held LW_EXCLUSIVE lock fail when ginFinishSplit

From a performance point of view, this doesn't matter. Incomplete split are extremely rare. For convenience, though, I added a new function specifically for handling these "leftover" incomplete splits as opposed to finishing a split that you just made, which performs the lock-upgrade. See attached. I think that helps with readability, and makes it less likely that we'll forget the lock-upgrade in the future if the insertion code is refactored.