Generalize chunk management hooks.

Add the "arena.<i>.chunk_hooks" mallctl, which replaces and expands on
the "arena.<i>.chunk.{alloc,dalloc,purge}" mallctls.  The chunk hooks
allow control over chunk allocation/deallocation, decommit/commit,
purging, and splitting/merging, such that the application can rely on
jemalloc's internal chunk caching and retaining functionality, yet
implement a variety of chunk management mechanisms and policies.

Merge the chunks_[sz]ad_{mmap,dss} red-black trees into
chunks_[sz]ad_retained.  This slightly reduces how hard jemalloc tries
to honor the dss precedence setting; prior to this change the precedence
setting was also consulted when recycling chunks.

Fix chunk purging.  Don't purge chunks in arena_purge_stashed(); instead
deallocate them in arena_unstash_purged(), so that the dirty memory
linkage remains valid until after the last time it is used.

This resolves #176 and #201.

ChangeLog

@@ -37,8 +37,7 @@ brevity.  Much more detail can be found in the git revision history:
     "opt.prof_thread_active_init", "prof.thread_active_init", and
     "thread.prof.active" mallctls.
   - Add support for per arena application-specified chunk allocators, configured
-    via the "arena<i>.chunk.alloc", "arena<i>.chunk.dalloc", and
-    "arena.<i>.chunk.purge" mallctls.
+    via the "arena.<i>.chunk_hooks" mallctl.
   - Refactor huge allocation to be managed by arenas, so that arenas now
     function as general purpose independent allocators.  This is important in
     the context of user-specified chunk allocators, aside from the scalability