Add the Mempage.aDataOfst field and use it in sqlite3BtreeMovetoUnpacked()

for about a 2 million cycle gain.

FossilOrigin-Name: bee94dc3510745ba2efa044e8f3299793cfc7e34

src/btreeInt.h

@@ -295,6 +295,7 @@ struct MemPage {
   u8 *aData;           /* Pointer to disk image of the page data */
   u8 *aDataEnd;        /* One byte past the end of usable data */
   u8 *aCellIdx;        /* The cell index area */
+  u8 *aDataOfst;       /* Same as aData for leaves.  aData+4 for interior */
   DbPage *pDbPage;     /* Pager page handle */
   u16 (*xCellSize)(MemPage*,u8*);             /* cellSizePtr method */
   void (*xParseCell)(MemPage*,u8*,CellInfo*); /* btreeParseCell method */