faild test with multiple HINT_CUSTOM_TREE_WALKERS

failed test when we have more than 1 walker HINT_CUSTOM_TREE_WALKERS ,
because walkSelectStatement in TreeWalkerChain saves the last hint _queryComponents result.
rename test
fix TreeWalkerChain.

Create walker object in foreach.
fix return walker
add iterator to tree walker chain
remove doc
Merge pull request #934 from shustrik/master

faild test with multiple HINT_CUSTOM_TREE_WALKERS