Create an implementaion of IntentIdGenerator

- Define interface to allocate IdBlock in IdBlockAllocator
- Implement IdBlockAllocatorBasedIntentIdGenerator
- Refactor IdBlock class

Change-Id: I21fa21ae625e3d7e137a7f846bb5a0c1bdb8df9a
diff --git a/src/main/java/net/onrc/onos/core/util/UnavailableIdException.java b/src/main/java/net/onrc/onos/core/util/UnavailableIdException.java
new file mode 100644
index 0000000..17c9df5
--- /dev/null
+++ b/src/main/java/net/onrc/onos/core/util/UnavailableIdException.java
@@ -0,0 +1,34 @@
+package net.onrc.onos.core.util;
+
+/**
+ * Represents that there is no available IDs.
+ */
+public class UnavailableIdException extends RuntimeException {
+
+    private static final long serialVersionUID = -2287403908433720122L;
+
+    /**
+     * Constructs an exception with no message and no underlying cause.
+     */
+    public UnavailableIdException() {
+    }
+
+    /**
+     * Constructs an exception with the specified message.
+     *
+     * @param message the message describing the specific nature of the error
+     */
+    public UnavailableIdException(String message) {
+        super(message);
+    }
+
+    /**
+     * Constructs an exception with the specified message and the underlying cause.
+     *
+     * @param message the message describing the specific nature of the error
+     * @param cause the underlying cause of this error
+     */
+    public UnavailableIdException(String message, Throwable cause) {
+        super(message, cause);
+    }
+}