[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.
from os.path import join
assert __file__.endswith(join('scripts', 'script2.py')), __file__
assert '__main__' == __name__, __name__
assert ['-v'] == __args__, __args__