Separate the crypto executor into its own module.

This allows it to be replaced for testing.

bramble-core/src/test/java/org/briarproject/bramble/test/TestCryptoExecutorModule.java

@@ -0,0 +1,21 @@
+package org.briarproject.bramble.test;
+
+import org.briarproject.bramble.api.crypto.CryptoExecutor;
+
+import java.util.concurrent.Executor;
+
+import javax.inject.Singleton;
+
+import dagger.Module;
+import dagger.Provides;
+
+@Module
+public class TestCryptoExecutorModule {
+
+	@Provides
+	@Singleton
+	@CryptoExecutor
+	Executor provideCryptoExecutor() {
+		return new ImmediateExecutor();
+	}
+}