Revision 1841225

* Doing a PROPFIND on a large collection e.g. 50.000 elements can easily
  consume 1 GB of memory as the subrequests and propdb pools are not
  destroyed and cleared after each element was handled.
  Do this now. There is one case in dav_get_props where elem->priv
  lives longer then the propdb pool. In this case allocate from r->pool.
  Furthermore also recycle propdb's which allows to clear the propdb's
  pools instead of destroying them and creating them again.