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.

src/chunk_dss.c

@@ -134,10 +134,10 @@ chunk_alloc_dss(arena_t *arena, void *new_addr, size_t size, size_t alignment,
 				dss_max = dss_next;
 				malloc_mutex_unlock(&dss_mtx);
 				if (cpad_size != 0) {
-					chunk_record(arena,
-					    &arena->chunks_szad_dss,
-					    &arena->chunks_ad_dss, false, cpad,
-					    cpad_size, false);
+					chunk_hooks_t chunk_hooks =
+					    CHUNK_HOOKS_INITIALIZER;
+					chunk_dalloc_wrapper(arena,
+					    &chunk_hooks, cpad, cpad_size);
 				}
 				if (*zero) {
 					JEMALLOC_VALGRIND_MAKE_MEM_UNDEFINED(