On the fsfs-format7 branch:  Refine caching strategies based on priorities.
With the introduction of a two-level design, we can now be much stricter
about priorties.

* subversion/libsvn_subr/cache-membuffer.c
  (find_entry): evict a relatively low-prio entry if an index bucket overflows
  (ensure_data_insertable_l2): be quick to reject / evict low-prio entries;
                               be a bit more elaborate in other cases


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1477166 13f79535-47bb-0310-9956-ffa450edef68