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/clock-skew.txt

@@ -0,0 +1,17 @@
+== One lock, two holders: what usingDbTime() prevents ==
+
+lockAtMostFor = 30s, both callers ask for the same lock name.
+
+JdbcTemplateLockProvider WITHOUT usingDbTime()
+  node with a correct clock  : acquired
+  node with a clock 40s fast : ACQUIRED -- two holders at the same time
+
+JdbcTemplateLockProvider WITH usingDbTime()
+  node with a correct clock  : acquired
+  node with a clock 40s fast : refused
+
+Without usingDbTime() the expiry comparison happens against the calling
+JVM's clock, so a node that is ahead by more than lockAtMostFor considers
+every live lock expired. With it, both the write and the comparison happen
+in the database, and there is only one clock in the system.
+