Fix and slightly simplify the debugging support in our membuffer cache.
It had been broken a while ago by the removal of the private sub-pool
from the cache frontends.

We use the same fixed-size buffer holding the type identifier extracted
from the cache prefix string everywhere we want a dev-visible type string.
To make that work, we double to buffer size, make the buffer always
available, be more intelligent when extracting the type part from the
original prefix and drop the old prefix duplicate.

* subversion/libsvn_subr/cache-membuffer.c
  (PREFIX_TAIL_LEN): Make always available and double size to accomodate
                     texts like "l2p-header".
  (get_prefix_tail): Make always available and trim the tail to just the
                     type ID in case we find the separating ':'.
  (DEBUG_CACHE_MEMBUFFER_INIT_TAG): Add pool parameter as the cache has no
                                    local pool instance anymore.  Also,
                                    access to common prefix buffer now.
  (entry_tag_t): Have a common, fixed-length prefix tail (=type) buffer.
  (svn_membuffer_cache_t,
   svn_membuffer_cache_get,
   svn_membuffer_cache_set,
   svn_membuffer_cache_get_partial,
   svn_membuffer_cache_set_partial): Provide the debug macro with a pool.
  (svn_membuffer_cache_get_info): Get the ID from the common buffer now.
  (svn_cache__create_membuffer_cache): Update initialization code.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1590455 13f79535-47bb-0310-9956-ffa450edef68