Rework the membuffer cache L2 promotion / eviction logic.  The goal is to
allow higher prio data to eventually replace low-prio entries which was not
the case with the prior randomized heuristics.

* subversion/libsvn_subr/cache-membuffer.c
  (ensure_data_insertable_l2): Accept low-prio entries only if they don't
                               encounter higher-prio ones; also remove them
                               quickly again.  Decide the remaining cases
                               solely based on priority and hits.  The aging
                               code will cause unused higher-prio entries
                               to be demoted and evicted eventually.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1580389 13f79535-47bb-0310-9956-ffa450edef68