Add a query to the stream API to determine if a stream supports
partial reads.

* subversion/include/svn_io.h
  (svn_stream_supports_partial_read): New.
* subversion/libsvn_subr/stream.c
  (svn_stream_supports_partial_read): Implement.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1569630 13f79535-47bb-0310-9956-ffa450edef68