Do not advance decay epoch when time goes backwards.

Instead, move the epoch backward in time.  Additionally, add
nstime_monotonic() and use it in debug builds to assert that time only
goes backward if nstime_update() is using a non-monotonic time source.

include/jemalloc/internal/nstime.h

@@ -31,9 +31,12 @@ void	nstime_imultiply(nstime_t *time, uint64_t multiplier);
 void	nstime_idivide(nstime_t *time, uint64_t divisor);
 uint64_t	nstime_divide(const nstime_t *time, const nstime_t *divisor);
 #ifdef JEMALLOC_JET
+typedef bool (nstime_monotonic_t)(void);
+extern nstime_monotonic_t *nstime_monotonic;
 typedef bool (nstime_update_t)(nstime_t *);
 extern nstime_update_t *nstime_update;
 #else
+bool	nstime_monotonic(void);
 bool	nstime_update(nstime_t *time);
 #endif
 

include/jemalloc/internal/private_symbols.txt

@@ -360,6 +360,7 @@ nstime_idivide
 nstime_imultiply
 nstime_init
 nstime_init2
+nstime_monotonic
 nstime_ns
 nstime_nsec
 nstime_sec