distributed pushed down joins - more features

update on latest accomplishments:

1. added support for filters (compare engine_condition_pushdown) on non-root operations, this in 2 flavors:
   
   • - constant/immediate filters that is provided after NdbQuery has been defined, these may not contains filter conditions referencing other NdbQueryOperation's,
   • - parameterized/linked filters, these type of programs must be provided when building the NdbQuery-object.
   
   The constant filters should be "easy" to integrate with ndbapi/mysqld, but the parameterized/linked is harder as we need to add new features to NdbScanFilter.
   
   Once this is integrated into mysqld, it will provide better performance for already pushable queries, see page 43 in my UC presentation (ref: filter).
   
   
2. added support for NdbQueryOpertation's referencing non-direct parent NdbQueryOperation's. This should also be quite easy to integrate into ndbapi/mysqld. This will mean that new types of queries will become pushable.
   
   
3. support for "to-many" joins is made so complete that it will get until we get an ndbapi for more elaborate testing. This will mean that lots of new queries will become pushable.
   

Note: all the additions has only been made inside ndb(mt)d so far, nothing new has (yet) been added to ndbapi/mysqld.

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And please provide feedback on preview release that we made!