JOIN_TAB::CONST_KEYS BUT NOT J_T::KEYS

JOIN_TAB::const_keys is a map of usable keys which have constant
predicates associated with it. Naturally, this is a a subset
of JOIN_TAB::keys which is a map of usable keys. 

Three issues are fixed with this patch:
1) In add_group_and_distinct_keys(), indexes usable for 
   resolving GROUP BY or DISTINCT were only added to const_keys.
   Now, these are also added to keys
2) Before this patch, add_group_and_distinct_keys() added 
   index bits to the const_keys map without checking if the
   field was even part of the table. Now the function bails
   out if GROUP BY or DISTINCT refers to a different table
   because no index on table 'a' can be used to resolve 
   GROUP BY/DISTINCT on a field from table 'b'.
3) Before this patch, JOIN::drop_unused_derived_keys() would
   call join_tab->keys.clear_all() but forget to do the same
   with join_tab->const_keys.