Followup to r1461689: reduce the overhead of maintainer builds.

Specifically, instead of a getenv() upon every cache access, do that just
once at cache creation and store the result.

Suggested by: stefan2

* subversion/libsvn_subr/cache.h
  (svn_cache__t.pretend_empty): New member.

* subversion/libsvn_subr/cache.c
  (svn_cache__get, svn_cache__iter, svn_cache__get_partial):
    Update the maintainer-mode-only "pretend empty?" condition.

* subversion/libsvn_subr/cache-inprocess.c
  (svn_cache__create_inprocess),
* subversion/libsvn_subr/cache-membuffer.c
  (svn_cache__create_membuffer_cache),
* subversion/libsvn_subr/cache-memcache.c
  (svn_cache__create_memcache):
    Initialise "pretend_empty".

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1476554 13f79535-47bb-0310-9956-ffa450edef68