Tighten pool usage in our standard reporter.

For not so obvious reasons, the pool passed to delta_dirs can take
a long time to get clean up eventually.  As a result, I got dynamic
peaks in memory usage of a whopping 20GB when running extreme tests
like 'svn-bench null-export' on the full kde.org repository.

With this fix, the peak is reduced to 100MB which is reaonably
"streamy" and will be reduced further by following commits.

* subversion/libsvn_repos/reporter.c
  (delta_dirs): Rename the existing subpool to iterpool.
                Create a subpool for all other allocations in this
                function and clean that up asap.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1564215 13f79535-47bb-0310-9956-ffa450edef68