ra_serf: Rotate connections when we stall instead of per file.
ra_serf: Rotate connections when we stall instead of per file.

This achieves a slightly better network distribution such that not all
pipelines tend to stall at the same time.

* subversion/libsvn_ra_serf/update.c
  (fetch_file): Don't touch cur_conn here.
  (finish_report): Rotate the cur_conn after each serf_context_run iteration.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion