Segregate some of optimizer rewrites into separate functions

Columnstore depends on a number of handy rewrites that MDB does, namely outter to inner join rewrite, IN into EXISTS conversion for IN + subquery. However MCS doesn't tolerate full JOIN::optimize processing b/c of an internal limitation in JOIN processing(the join graph must be acyclic).
We need a new dedicated optimize() method in Select Handler that must replace JOIN::optimize() call. This method must be able to call dedicated functions that can apply rewrites listed above.