feat: add missing indexes to the oracle ddl script (#503)

## Add missing indexes to the oracle ddl script

Every example for other databases (Postgres, Mssql, Mysql) contains
indexes on `execution_time` and `last_heartbeat` while for Oracle DB
they are missing.

I also expanded `DbUtiils` method to allow splitting sql statements from
an sql file. `jdbcRunner.execute(statement, NOOP)` can run only one
statement at a time for Oracle Db.

PS: two tests for oracle db are failing on master, is that expected?

## Reminders
- [x] Added/ran automated tests
- [x] Update README and/or examples
    - no need
- [x] Ran `mvn spotless:apply`

---
cc @kagkarlsson

db-scheduler/src/test/java/com/github/kagkarlsson/scheduler/DbUtils.java

@@ -23,13 +23,25 @@ public static void clearTables(DataSource dataSource) {
   }
 
   public static Consumer<DataSource> runSqlResource(String resource) {
+    return runSqlResource(resource, false);
+  }
+
+  public static Consumer<DataSource> runSqlResource(String resource, boolean splitStatements) {
     return dataSource -> {
       final JdbcRunner jdbcRunner = new JdbcRunner(dataSource);
       try {
         final String statements =
             CharStreams.toString(
                 new InputStreamReader(DbUtils.class.getResourceAsStream(resource)));
-        jdbcRunner.execute(statements, NOOP);
+        if (splitStatements) {
+          for (String statement : statements.split(";")) {
+            if (!statement.trim().isEmpty()) {
+              jdbcRunner.execute(statement, NOOP);
+            }
+          }
+        } else {
+          jdbcRunner.execute(statements, NOOP);
+        }
       } catch (IOException e) {
         throw new RuntimeException(e);
       }

db-scheduler/src/test/java/com/github/kagkarlsson/scheduler/compatibility/Oracle11gCompatibilityTest.java

@@ -44,7 +44,7 @@ static void initSchema() {
     pooledDatasource = new HikariDataSource(hikariConfig);
 
     // init schema
-    DbUtils.runSqlResource("/oracle_tables.sql").accept(pooledDatasource);
+    DbUtils.runSqlResource("/oracle_tables.sql", true).accept(pooledDatasource);
   }
 
   @BeforeEach

db-scheduler/src/test/resources/oracle_tables.sql

@@ -12,5 +12,7 @@ create table scheduled_tasks
     last_heartbeat       TIMESTAMP(6) WITH TIME ZONE,
     version              NUMBER(19, 0),
     PRIMARY KEY (task_name, task_instance)
-)
+);
 
+CREATE INDEX scheduled_tasks__execution_time__idx on scheduled_tasks(execution_time);
+CREATE INDEX scheduled_tasks__last_heartbeat__idx on scheduled_tasks(last_heartbeat);