Resolves #249 - removes the state monitor runnable from the channel

state and replaces it with a processing chain heartbeat that is generated
by the sample source.

Previously, the state monitor was running on a separate thread and this
was causing multi-threading issues.  With this update, all channel state
operations (and all module operations) occur on the source's buffer
processing thread.

src/channel/heartbeat/IHeartbeatProvider.java

@@ -0,0 +1,28 @@
+/*******************************************************************************
+ * sdrtrunk
+ * Copyright (C) 2014-2017 Dennis Sheirer
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ *
+ ******************************************************************************/
+package channel.heartbeat;
+
+import sample.Listener;
+
+public interface IHeartbeatProvider
+{
+    public void setHeartbeatListener(Listener<Heartbeat> listener);
+
+    public void removeHeartbeatListener();
+}