Renamed KeyValueBytes to ItemBytes and made it work.

cbugg: bug-397

The ItemBytes stat now survives a server restart.  It represents the
total key + value + metadata number of bytes needed to store items,
not including any gkvlite tree nodes or the key-index.

Some extra bookkeeping metadata (vbucket state changes) are also
counted as part of the ItemBytes, but I don't yet have a nice way
address that yet.