Revision 1470936

Significantly reduce the number of open files during a typical update
over ra_serf by using lazy-opening streams to delay opening until the
HTTP response is received.  This changes the new-in-1.8 lazy-open API.

* subversion/include/svn_io.h
  (svn_stream_lazyopen_create): Add open_on_close boolean parameter.

* subversion/libsvn_subr/stream.c
  (struct lazyopen_baton_t): Add open_on_close boolean member.
  (close_handler_lazyopen): Open if required.
  (svn_stream_lazyopen_create): Add open_on_close boolean parameter.

* subversion/libsvn_wc/update_editor.c
  (lazy_open_source, lazy_open_target, struct lazy_target_baton): New.
  (apply_textdelta): Use lazy-open streams for source and target.

* subversion/libsvn_wc/adm_ops.c
  (svn_wc__get_pristine_contents_by_checksum): Adjust call.