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/three-replicas-locked.txt

@@ -0,0 +1,37 @@
+profile=locked  replicas=3  run-for=30s  job rate=3s  job duration=PT0.2S  replica-3 clock skew=0s
+
+-- every execution, in order --
+ instance_id |  started_at  
+-------------+--------------
+ replica-2   | 22:49:29.233
+ replica-3   | 22:49:31.931
+ replica-3   | 22:49:34.925
+ replica-3   | 22:49:37.934
+ replica-3   | 22:49:40.934
+ replica-3   | 22:49:43.925
+ replica-3   | 22:49:46.934
+(7 rows)
+
+-- executions per replica --
+ instance_id | count 
+-------------+-------
+ replica-2   |     1
+ replica-3   |     6
+(2 rows)
+
+-- pairs of executions that overlapped (a second replica started while the first was still working) --
+ overlapping_pairs 
+-------------------
+                 0
+(1 row)
+
+ first | first_at | second | second_at 
+-------+----------+--------+-----------
+(0 rows)
+
+-- the lock row --
+     name      | locked_by |  locked_at   |  lock_until  
+---------------+-----------+--------------+--------------
+ nightlyReport | unknown   | 17:19:46.921 | 17:19:47.921
+(1 row)
+