Moved message verification into a separate thread pool.

api/net/sf/briar/api/protocol/VerificationExecutor.java

@@ -0,0 +1,15 @@
+package net.sf.briar.api.protocol;
+
+import static java.lang.annotation.ElementType.PARAMETER;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import com.google.inject.BindingAnnotation;
+
+/** Annotation for injecting the executor for message verification tasks. */
+@BindingAnnotation
+@Target({ PARAMETER })
+@Retention(RUNTIME)
+public @interface VerificationExecutor {}