Following up on r1555716, r1556343 and the reversion of these in r1556933,
implement yet another create body to memory spool implementation. This time
using serf buckets to allow releasing all ram when we have to switch to a
file.

* subversion/libsvn_ra_serf/update.c
  (body_create_baton_t): New typedef.
  (report_context_t): Update variables.
  (MAX_BODY_IN_RAM): New define.
  (body_allocate_all,
   serf_free_no_error
   body_write_fn,
   body_done_fn): New functions.

  (set_path,
   delete_path,
   link_path): Write to stream. Once we can assume the serf bucket length
     support we can probably switch to writing buckets directly.

  (create_update_report_body): Implement passing body buffer.
  (finish_report): Update for stream. Remove mmap specifics.
  (make_update_reporter): Initialize body collector. Write to stream.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1557599 13f79535-47bb-0310-9956-ffa450edef68