Introduce a @Scheduler annotation

and make sure work is offloaded to an executor, so exceptions can be
caught.

briar-api/src/org/briarproject/api/system/Scheduler.java

@@ -0,0 +1,25 @@
+package org.briarproject.api.system;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import javax.inject.Qualifier;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.ElementType.PARAMETER;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+/**
+ * Annotation for injecting a scheduled executor service
+ * that can be used to schedule the execution of tasks.
+ * <p>
+ * The service should <b>only</b> be used for running tasks on other executors
+ * at scheduled times.
+ * No significant work should be run by the service itself!
+ */
+@Qualifier
+@Target({FIELD, METHOD, PARAMETER})
+@Retention(RUNTIME)
+public @interface Scheduler {
+}