Make the svn_ra_get_mergeinfo streamy for ra_serf.
Make the svn_ra_get_mergeinfo streamy for ra_serf.

All the groundwork has been done in the repos and FS layers and we
only need to use the new callback-based API.  The current pool usage
in brigades is opaque to this code.  So, the memory usage might still
be unbound but it will still be noticibly less than before.

* subversion/svnserve/serve.c
  (mergeinfo_receiver_baton_t,
   send_mergeinfo_starting_sequence,
   mergeinfo_receiver):  The new callback code.
  (get_mergeinfo): Bump to using the new API.

git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1780845 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion