In libsvn_ra_serf: Construct all svn_ra_serf__handler_t instances
via one constructor function.

* subversion/libsvn_ra_serf/commit.c
  (checkout_node): Allocate handler in scratch pool.
  (proppatch_resource,
   open_root,
   delete_entry,
   add_directory,
   add_file,
   close_file,
   close_edit): Use constructor.

* subversion/libsvn_ra_serf/inherited_props.c
  (svn_ra_serf__get_inherited_props): Remove unneeded set.

* subversion/libsvn_ra_serf/lock.c
  (svn_ra_serf__unlock): Use constructor.

* subversion/libsvn_ra_serf/options.c
  (svn_ra_serf__probe_proxy): Use constructor.

* subversion/libsvn_ra_serf/ra_serf.h
  (svn_ra_serf__handler_t): Extend comment.
  (svn_ra_serf__create_handler): New function.

* subversion/libsvn_ra_serf/update.c
  (fetch_file): Use constructor.
  (finish_report): Use constructor.
  (svn_ra_serf__get_file): Use constructor.
  (handle_response): Add comment.
  (svn_ra_serf__create_handler): New function.
  (svn_ra_serf__create_expat_handler): Use svn_ra_serf__create_handler.
* subversion/libsvn_ra_serf/util.c


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1554019 13f79535-47bb-0310-9956-ffa450edef68