Refactoring to batch (#181)

The rationale of this pull request is to

* have more maintainable and granular code
* not maintain the custom made job engine
* allow the users to customize the defaults of the releaser more easy
* allow the users to create their own steps without the need to change any existing code
* allow the users to fully change the flows and tasks logic
* abstract underlying batch mechanism (Spring Batch) so it doesn't leak to production code
* allow parallelization of the release process and release tasks

releaser-spring/src/main/java/releaser/internal/tasks/ConditionalOnDefaultFlowEnabled.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2013-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package releaser.internal.tasks;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
+
+/**
+ * Allows to disable the default flow and gives an option to manually setup the whole
+ * releaser flow.
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Target({ ElementType.TYPE, ElementType.METHOD })
+@Documented
+@ConditionalOnProperty(value = "releaser.flow.default-enabled", matchIfMissing = true)
+public @interface ConditionalOnDefaultFlowEnabled {
+
+}