WL#4763: Prototype of simple parallel query

The idea here is to do the following.

1) Make a small number of item classes reentrant.
2) Make a walker to see if item tree is reentrant.
3) Make the JOIN_TAB reentrant by small modifications
(not the full split of its data).
4) Rely on cursor class in handler interface
developed for Foreign Key
5) Simple threading mechanism
6) Test with a small number of queries

The purpose of this task is to discover if the
choosen method is viable and it's not yet the
purpose to go for a full-fledged Parallel Query
implementation.