query handlers refactoring

instead of inheritance handler are called now in their creation order
QueryHandler become a normal base class - no longer singleton
A global variable holds pointer to recently created handler, each newly
createted handler saves pointer to previous handler and overwrites the
global variable. When postgres calls hooks, then methods of handler are
called in order form newest handler to oldest.
RootQueryHandler class added - base class for handlers which needs to
distinguish between root and sub queries.