Make a compatibility macro for 'apr_hash_clear()', to be used when APR <
1.3.0, similar to the compatibility macro 'apr_array_clear' that we already
have for the same reason, so that we can use it regardless of which APR we
compile against.  Call apr_hash_clear() instead of svn_hash__clear(),
everywhere (that is, in a grand total of four files).

Change the signature of svn_hash__clear() to match apr_hash_clear() and use
it to implement the compatibility macro.  It now uses the hash's own private
iterator (which we don't generally use in Subversion), like APR's
apr_hash_clear() does.

* subversion/include/private/svn_subr_private.h
  (svn_hash__clear): Remove this declaration.

* subversion/include/private/svn_dep_compat.h
  (svn_hash__clear): Declare (the new signature) when APR < 1.3.0.
  (apr_hash_clear): New macro, when APR < 1.3.0; calls svn_hash__clear.

* subversion/libsvn_subr/hash.c
  (svn_hash__clear): Remove the pool argument, and return void.

* subversion/libsvn_ra_serf/serf.c
  (svn_ra_serf__get_dir): Call apr_hash_clear() instead of svn_hash__clear().

* subversion/libsvn_wc/adm_ops.c
  (svn_wc_process_committed_queue2): Same.

* subversion/svn/notify.c
  (svn_cl__notifier_reset_conflict_stats): Same.

* subversion/svnrdump/dump_editor.c
  (do_dump_props, close_directory, close_file): Same.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1459058 13f79535-47bb-0310-9956-ffa450edef68