ITS#2183 IDL stack slab

[hyc@highlandsun.com]

Thinking about this some more; it can be greatly simplified. Instead of
having a sequence of buckets of increasing length, just use a single
fixed-size buffer per thread. This would also avoid the need for any locking
on the structures. (See the use of ldap_pvt_thread_pool_getkey/setkey in
cache.c.) I can code this up pretty quickly for a test.

  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun
  http://www.symas.com               http://highlandsun.com/hyc
  Symas: Premier OpenSource Development and Support