Use nanoTime instead of currentTimeMillis for elapsed time

System/currentTimeMillis is not guaranteed to progress monotonically.
NTP shifts, leap seconds, and manual system time changes can all cause
elapsed time calculations to be incorrect if you use
currentTimeMillis.

nanoTime is always calculated against a fixed point and proceeds
monotonically.

https://go.googlesource.com/proposal/+/master/design/12914-monotonic.md
has a good discussion on monotonic time.

Closes: clj-commons#21

Author: danielcompton

src/durable_queue.clj

@@ -562,10 +562,10 @@
              (while (.get ref)
                (when-let [q (.get ref)]
                  (try
-                   (let [start (System/currentTimeMillis)]
+                   (let [start (System/nanoTime)]
                      (fsync q)
-                     (let [end (System/currentTimeMillis)]
-                       (Thread/sleep (max 0 (- fsync-interval (- end start))))))
+                     (let [end (System/nanoTime)]
+                       (Thread/sleep (max 0 (- (* 1000000 fsync-interval) (- end start))))))
                    (catch Throwable e
                      )))))))
 
@@ -759,16 +759,16 @@
   "Returns a lazy sequence of tasks that can be consumed in `interval` milliseconds.  This will
    terminate after that time has elapsed, even if there are still tasks immediately available."
   [qs q-name interval]
-  (let [now (System/currentTimeMillis)]
+  (let [now (System/nanoTime)]
     (lazy-seq
-      (let [now' (System/currentTimeMillis)
-            remaining (- interval (- now' now))]
+      (let [now' (System/nanoTime)
+            remaining (- (* 1000000 interval) (- now' now))]
         (when (pos? remaining)
           (let [task (take! qs q-name remaining ::none)]
             (when-not (= ::none task)
               (cons
                 task
-                (interval-task-seq qs q-name (- interval (- (System/currentTimeMillis) now)))))))))))
+                (interval-task-seq qs q-name (- (* 1000000 interval) (- (System/nanoTime) now)))))))))))
 
 (defn complete!
   "Marks a task as complete."