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-unlocked.txt

@@ -0,0 +1,60 @@
+profile=unlocked  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-1   | 22:48:54.564
+ replica-3   | 22:48:54.752
+ replica-2   | 22:48:55.052
+ replica-1   | 22:48:57.551
+ replica-3   | 22:48:57.736
+ replica-2   | 22:48:58.030
+ replica-1   | 22:49:00.551
+ replica-3   | 22:49:00.736
+ replica-2   | 22:49:01.030
+ replica-1   | 22:49:03.551
+ replica-3   | 22:49:03.736
+ replica-2   | 22:49:04.030
+ replica-1   | 22:49:06.551
+ replica-3   | 22:49:06.736
+ replica-2   | 22:49:07.030
+ replica-1   | 22:49:09.551
+ replica-3   | 22:49:09.736
+ replica-2   | 22:49:10.030
+ replica-1   | 22:49:12.551
+ replica-3   | 22:49:12.736
+ replica-2   | 22:49:13.030
+ replica-1   | 22:49:15.551
+ replica-3   | 22:49:15.736
+ replica-2   | 22:49:16.030
+(24 rows)
+
+-- executions per replica --
+ instance_id | count 
+-------------+-------
+ replica-1   |     8
+ replica-2   |     8
+ replica-3   |     8
+(3 rows)
+
+-- pairs of executions that overlapped (a second replica started while the first was still working) --
+ overlapping_pairs 
+-------------------
+                 8
+(1 row)
+
+   first   |   first_at   |  second   |  second_at   
+-----------+--------------+-----------+--------------
+ replica-1 | 22:48:54.564 | replica-3 | 22:48:54.752
+ replica-1 | 22:48:57.551 | replica-3 | 22:48:57.736
+ replica-1 | 22:49:00.551 | replica-3 | 22:49:00.736
+ replica-1 | 22:49:03.551 | replica-3 | 22:49:03.736
+ replica-1 | 22:49:06.551 | replica-3 | 22:49:06.736
+ replica-1 | 22:49:09.551 | replica-3 | 22:49:09.736
+(6 rows)
+
+-- the lock row --
+ name | locked_by | locked_at | lock_until 
+------+-----------+-----------+------------
+(0 rows)
+