For issue #4450 "Windows checkouts much slower then Unix" create a private api
around installing a file, to prepare for an optimized Windows implementation
that reduces a lot of disk IO delays during update and checkout.

* subversion/include/private/svn_io_private.h
  (svn_stream__create_for_install): New function.
  (svn_stream__install_stream): New function.

* subversion/libsvn_subr/stream.c
  (install_baton_t): New struct.
  (svn_stream__create_for_install): New function.
  (svn_stream__install_stream): New function.

* subversion/libsvn_wc/workqueue.c
  (includes): Add private/svn_io_private.h.
  (run_file_install): Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1559758 13f79535-47bb-0310-9956-ffa450edef68