When constructing the key prefix that we use for key space separation
in FSFS caches,  make sure that the ":" delimiter is not found in any
of the key parts.  Otherwise, we might be able to construct the same
prefix using a different combination of elements.

Normalizing those parts that potentially need it must not limit their
value space, however, as this would again allow for different element
values to result in the same key prefix.

* subversion/libsvn_fs_fs/caching.c
  (normalize_namespace): new utility function
  (read_config,
   svn_fs_fs__initialize_caches): normalize elements of the key prefix


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1462907 13f79535-47bb-0310-9956-ffa450edef68