replace QuartzScheduler with a more lightweight scheduler

The new scheduler is based on the cron-utils package and uses the same cron syntax as quartz.
CronScheduler was mainly introduced, because of a ClassLoader leak with the old Quartz implementation.
The leak comes from shiros use of InheriatableThreadLocal in combination with the WorkerThreads of Quartz.
CronScheduler uses a ThreadFactory which clears the Shiro context before a new Thread is created (see CronThreadFactory).

scm-webapp/src/main/java/sonia/scm/schedule/CronExpression.java

@@ -0,0 +1,59 @@
+package sonia.scm.schedule;
+
+import com.cronutils.model.Cron;
+import com.cronutils.model.CronType;
+import com.cronutils.model.definition.CronDefinition;
+import com.cronutils.model.definition.CronDefinitionBuilder;
+import com.cronutils.model.time.ExecutionTime;
+import com.cronutils.parser.CronParser;
+
+import java.time.Clock;
+import java.time.Duration;
+import java.time.ZonedDateTime;
+import java.util.Optional;
+
+final class CronExpression {
+
+  private final Clock clock;
+  private final String expression;
+  private final ExecutionTime executionTime;
+
+  CronExpression(String expression) {
+    this(Clock.systemUTC(), expression);
+  }
+
+  CronExpression(Clock clock, String expression) {
+    this.clock = clock;
+    this.expression = expression;
+    executionTime = createExecutionTime(expression);
+  }
+
+  boolean shouldRun(ZonedDateTime time) {
+    ZonedDateTime now = ZonedDateTime.now(clock);
+    return time.isBefore(now);
+  }
+
+  Optional<ZonedDateTime> calculateNextRun() {
+    ZonedDateTime now = ZonedDateTime.now(clock);
+    Optional<ZonedDateTime> nextExecution = executionTime.nextExecution(now);
+    if (nextExecution.isPresent()) {
+      ZonedDateTime next = nextExecution.get();
+      if (Duration.between(now, next).toMillis() < 1000) {
+        return executionTime.nextExecution(now.plusSeconds(1L));
+      }
+    }
+    return nextExecution;
+  }
+
+  private ExecutionTime createExecutionTime(String expression) {
+    CronDefinition cronDefinition = CronDefinitionBuilder.instanceDefinitionFor(CronType.QUARTZ);
+    CronParser parser = new CronParser(cronDefinition);
+    Cron cron = parser.parse(expression);
+    return ExecutionTime.forCron(cron);
+  }
+
+  @Override
+  public String toString() {
+    return expression;
+  }
+}