Update the Cell class in btree_format to support decoding delta page cells. Leaf page cells have a special format as defined in cell.h

#define WT_DELTA_LEAF_VALUE_FORMAT WT_UNCHECKED_STRING(uB)

Since we're using the set 'u' type we also have to consider (from the docs):

The 'u' type is for raw byte arrays: if it appears at the end of a format string (including in the default "u" format for untyped tables), the size is not stored explicitly. When 'u' appears within a format string, the size is stored as a 32-bit integer in the same byte order as the rest of the format string, followed by the data.