Make the membuffer cache accept even large items if they are important
enough.  By allowing large items to be directly inserted into L2, we
can accept larger objects than before.  The item size limit is now
about 4 times the previous value.

This almost exclusively benefits directories that those are the only
large non-low-prio objects that we handle.  Since cache eviction rules
still apply, it may take multiple attempts (i.e. accesses) until L2
has been cleaned out enough to leave room for the new object.  But
frequently used, large directories will eventually get cached.

* subversion/libsvn_subr/cache-membuffer.c
  (select_level): New utility that determines where items should
                  be inserted.
  (membuffer_cache_set_internal): Let the new utility function select the
                                  cache level for the new item instead of
                                  always using L1.
  (svn_membuffer_cache_is_cachable): Indicate that we also allow for
                                     large important items to be cached.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1545948 13f79535-47bb-0310-9956-ffa450edef68