Refactor glass backend key comparison

Refactor glass backend key comparison

Instead of comparing Key objects, compare BItem and LeafItem objects by
their keys, which is mainly an enabler for future changes.

Instead of operator== and operator<, provide a compare() function
which returns negative, 0 or positive, like strcmp(), memcmp() and
std::string::compare().  This allows us to avoid a final compare to
check for equality in GlassTable::find_in_leaf(), and to terminate
early if the binary chop hits the exact entry.