Avoid membuffer cache thrashing by large objects, e.g. large fulltexts.
Only very critical information shall go into cache even if the object
is larger than the whole L1.

This has become an issue since we elevated fulltexts from low prio to
default caching prio.

* subversion/libsvn_subr/cache-membuffer.c
  (select_level,
   svn_membuffer_cache_is_cachable): Allow only higher prio items to skip
                                     L1 and go directly into the larger L2
                                     (evicting everything in their wake).

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1590986 13f79535-47bb-0310-9956-ffa450edef68