Add the scheduling module

Three replicas of one application against one PostgreSQL database, proving
duplicate @Scheduled execution and then removing it with ShedLock: 24
executions where 8 were due, then 7 for 7 ticks.

Also measured: @SchedulerLock without @EnableSchedulerLock does nothing and
warns about nothing; spring.task.scheduling.pool.size=1 does not starve a
fixedRate job but delays it and fires 35 of 40 executions in a burst; and a
node whose clock is 40 seconds fast takes a live lock unless the provider
uses usingDbTime().

scheduling/docs/output/scheduler-pool-4.txt

@@ -0,0 +1,18 @@
+== spring.task.scheduling.pool.size=4 ==
+
+over 8 seconds, three @Scheduled methods on one application
+  slow()     fixedRate 2000 ms, sleeps 1800 ms : 5 executions
+  fast()     fixedRate  200 ms                 : 41 executions
+  throwing() fixedRate  300 ms, always throws  : 27 executions
+  distinct scheduler threads                   : 4 [scheduling-1, scheduling-2, scheduling-3, scheduling-4]
+
+  longest gap between two fast() executions    : 200 ms   (200 ms was the schedule)
+  fast() executions that started within 20 ms
+  of the previous one (the catch-up burst)     : 0
+
+throwing() kept its schedule after every failure. Spring wraps a scheduled
+method in TaskUtils.LOG_AND_SUPPRESS_ERROR_HANDLER, so each exception is
+logged by o.s.s.s.TaskUtils$LoggingErrorHandler as "Unexpected error
+occurred in scheduled task" and then discarded. A raw
+ScheduledExecutorService would have cancelled the task at the first one.
+