Add DfsIndexIterator.

Summary:
 - provides an iterator which performs a DFS walk through the index,
 starting from a node and iterating through all subexpressions
 corresponding to the respective node.
 - it replaces this logic in TmpIndex::exportToMemsector and
 in search context qvar resolution.

Test Plan: make test

Reviewers: radu.hambasan

Reviewed By: radu.hambasan

Differential Revision: http://phab.code4fun.de/D82