Remove the datatype from the Blob

As part of fixing MB-22163 we're keeping a copy of the datatype inside the StoredValue (by using one of the available pad bytes), so we don't really need to keep it in the blob. This could save some memory on a per-item base (depending on the underlying allocation boundaries).

Note that ep-engine store the datatype in the blob field on disk so we need to either fix how we store it on disk, or strip it off / put it back in when items go between disk and memory.