Refactor decay-related function parametrization.

Refactor most of the decay-related functions to take as parameters the
decay_t and associated extents_t structures to operate on.  This
prepares for supporting both lazy and forced purging on different decay
schedules.

include/jemalloc/internal/arena_structs_b.h

@@ -174,13 +174,6 @@ struct arena_s {
 	 */
 	size_t			nactive;
 
-	/*
-	 * Decay-based purging state.
-	 *
-	 * Synchronization: lock.
-	 */
-	arena_decay_t		decay;
-
 	/*
 	 * Extant large allocations.
 	 *
@@ -199,6 +192,13 @@ struct arena_s {
 	extents_t		extents_cached;
 	extents_t		extents_retained;
 
+	/*
+	 * Decay-based purging state.
+	 *
+	 * Synchronization: internal.
+	 */
+	arena_decay_t		decay;
+
 	/*
 	 * True if a thread is currently executing arena_purge_to_limit().
 	 *