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/src/main/resources/schema.sql

@@ -0,0 +1,18 @@
+-- Both tables are created by every replica at startup, so every statement is IF NOT EXISTS.
+-- The shedlock table's shape is fixed by ShedLock: name, lock_until, locked_at, locked_by.
+-- lock_until and locked_at must be timestamps; with usingDbTime() they are written by the
+-- database, so their precision and time zone are the database's, not the JVM's.
+create table if not exists shedlock (
+    name       varchar(64)  not null,
+    lock_until timestamp    not null,
+    locked_at  timestamp    not null,
+    locked_by  varchar(255) not null,
+    primary key (name)
+);
+
+create table if not exists job_execution (
+    id          serial primary key,
+    job_name    varchar(64)  not null,
+    instance_id varchar(64)  not null,
+    started_at  timestamp    not null
+);