Clean up background thread sleep computation

Isolate the computation of purge interval from background thread logic and
move into more suitable file.

include/jemalloc/internal/decay.h

@@ -3,6 +3,8 @@
 
 #include "jemalloc/internal/smoothstep.h"
 
+#define DECAY_UNBOUNDED_TIME_TO_PURGE ((uint64_t)-1)
+
 /*
  * The decay_t computes the number of pages we should purge at any given time.
  * Page allocators inform a decay object when pages enter a decay-able state
@@ -146,4 +148,13 @@ void decay_reinit(decay_t *decay, nstime_t *cur_time, ssize_t decay_ms);
 bool decay_maybe_advance_epoch(decay_t *decay, nstime_t *new_time,
     size_t current_npages);
 
+/*
+ * Calculates wait time until at least npages_threshold pages should be purged.
+ *
+ * Returns number of nanoseconds or DECAY_UNBOUNDED_TIME_TO_PURGE in case of
+ * indefinite wait.
+ */
+uint64_t decay_ns_until_purge(decay_t *decay, size_t npages_current,
+    uint64_t npages_threshold);
+
 #endif /* JEMALLOC_INTERNAL_DECAY_H */