Remove string digesting from PersistitKeyValueTarget

Looks like it was originally added to facilitate sort/distinct on (very) long strings when we PersistitSorter was the main path. Somehow it leaked out into index rows as well. Since it smashes the string after 240 bytes, this breaks covering indexes.

Instead of trying to catch all ramifications, just remove it. Note that MergeJoinSorter isn't vulnerable to the same 2k limit so Sort_General_LargeKeyIT still passes.

Unfortunately this is a data format change, as any index on a string >= 255 characters is affected. I could be convinced *not* to bump dataVersion as this doesn't actually change how they are interpreted, just stops breaking them on the way in.