We should enable revprop caching only if the respective system does 
support it efficiently.

* subversion/svnserve/svnserve.c:
  (main): set "enable revprop caching" config flag only to "enable if 
          efficient" ("2") instead of "enable unconditionally" ("1")
* subversion/mod_dav_svn/repos.c:
  (get_resource): ditto

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543593 13f79535-47bb-0310-9956-ffa450edef68