Change the RA tunnel callbacks so that they no longer refer to types
defined in the svn_ra_svn.h header.

* subversion/include/svn_ra.h
  (struct svn_ra_svn_conn_st): Remove forward declaration.
  (svn_ra_open_tunnel_func_t): Replace the conn output parameter with
   request and response file handles.
* subversion/libsvn_ra_svn/client.c (open_session): Update to match
   the changed open-tunnel callback signature, and creat the ra_svn
   session here.

* subversion/tests/libsvn_ra/ra-test.c: Do not include svn_ra_svn.h.
  (open_tunnel): Fix signature and implementation.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1531610 13f79535-47bb-0310-9956-ffa450edef68