ra_serf: Further decrease memory while doing a svnsync.

This avoid increased memory usage based on the number of total directories
opened and properties changed.  The depth of directories opened at the same
time can still impact the memory usage though.

* subversion/libsvn_ra_serf/replay.c
  (replay_context_t): Remove file_pool member.
  (push_state): Generate a new pool for each replay_info_t.
  (start_replay): Remove file_pool and use info->pool instead (destroying
    info->pool when closing the file or dir).
  (end_replay): Use info->pool instead of file_pool and destroy info->pool
    after the finishing setting the property.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x-svnsync-serf-memory@1515269 13f79535-47bb-0310-9956-ffa450edef68