Revision 1700799

[Combines r1698359 and r170078 into a single commit for better review.]
Introduce a stream wrapper object that adds buffering support to any
readable stream.  Use it on the stdin streams in our CL tools. 

As it turns out, parsing data from a stdin byte-by-byte incurs a
massive overhead of 100% internal and 300% system load over a buffered
stream.  'svnadmin load-revprops' sees a 5 times speedup if all data
is in OS disc caches.  This is a realistic assumption in a "final sync
and switch over to new repository" scenario.

The other 2 uses of stdin either have less data to process (svnfsfs
load-index) or parse only a small fraction of the stream (svnadmin load).

* subversion/include/svn_io.h
  (svn_stream_wrap_buffered_read): Declare the new stream constructor API.

* subversion/libsvn_subr/stream.c
  (read_handler_buffering_wrapper,
   data_available_handler_buffering_wrapper,
   is_buffered_handler_buffering_wrapper): Internal logic of the new
                                           stream object.
  (svn_stream_wrap_buffered_read): New constructor implementation.

* subversion/svnadmin/svnadmin.c
  (subcommand_load_revprops): Wrap the stdin stream.

* subversion/svnfsfs/load-index-cmd.c
  (subcommand__load_index): Same.

* subversion/tests/libsvn_subr/stream-test.c
  (struct stream_baton_t,
   read_handler,
   data_available_handler,
   create_test_read_stream): New configurable test read stream.
  (expect_line_content,
   test_stream_buffered_wrapper): New test for the new wrapper stream. 
  (test_funcs): Register the new test.