Introduce the concept of namespaces to FSFS caching.  We already use
"prefixes" to separate keys from different repos.  Now, we can add
another element to the prefix and we will do so if we need to ensure
that all data get read from disk at least initially.

Make all verification code use a unique cache namespace.

* subversion/include/svn_fs.h
  (SVN_FS_CONFIG_FSFS_CACHE_NS): declare new config option

* subversion/libsvn_fs_fs/caching.c
  (read_config): read namespace from config
  (svn_fs_fs__initialize_caches): add namespace to prefix

* subversion/libsvn_fs_fs/fs_fs.c
  (verify_as_revision_before_current_plus_plus): use unique namespace
   instead of disabling caches altogether

* subversion/svnadmin/svnadmin.c
  (open_repos): use unique namespace for all ops

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1462436 13f79535-47bb-0310-9956-ffa450edef68