Implement decay-based unused dirty page purging.

This is an alternative to the existing ratio-based unused dirty page
purging, and is intended to eventually become the sole purging
mechanism.

Add mallctls:
- opt.purge
- opt.decay_time
- arena.<i>.decay
- arena.<i>.decay_time
- arenas.decay_time
- stats.arenas.<i>.decay_time

This resolves #325.

include/jemalloc/internal/ctl.h

@@ -35,6 +35,7 @@ struct ctl_arena_stats_s {
 	unsigned		nthreads;
 	const char		*dss;
 	ssize_t			lg_dirty_mult;
+	ssize_t			decay_time;
 	size_t			pactive;
 	size_t			pdirty;
 	arena_stats_t		astats;