[rset] Use RQL cache in syntax_tree method This avoid several unnecessary parsings per HTTP request, since in most case rset's rql is in the rql cache (since it has just been executed), while yapps based parsing is known to be slow.

pytest
http://hg.logilab.org/master/logilab/common/archive/default.tar.bz2#egg=logilab-common
http://hg.logilab.org/master/yams/archive/default.tar.bz2#egg=yams