Sub-optimally re-read item so NumBytes() works.

The right fix is to remember the value length in each node instead of
re-reading the entire item in order to get the right item.NumBytes().
TestSlabStoreRandom().
Const itemLoc_hdrLength.
Added itemLoc.NumBytes() to avoid extra val read's.

When possible, itemLoc.NumBytes() favors simple math on the known
persisted bytes length minus the itemLoc header length, allowing us to
avoid a disk read.