Reworking thread termination logic.

Turns out the finish methods weren't called correctly, caused by a
mess up with method names which all sounded too similar and the wrong
one ended up being called. I've reworked this by changing the
thread/writer/reader interfaces, which actually also simplifies them
by getting rid of the requirement for writer backends to call their
parent methods (i.e., less opportunity for errors).

This commit also includes the following (because I noticed the problem
above when working on some of these):

     - The ASCII log writer now includes "#start <timestamp>" and
      "#end <timestamp> lines in the each file. The latter supersedes
      Bernhard's "EOF" patch.

      This required a number of tests updates. The standard canonifier
      removes the timestamps, but some tests compare files directly,
      which doesn't work if they aren't printing out the same
      timestamps (like the comm tests).

     - The above required yet another change to the writer API to
       network_time to methods.

     - Renamed ASCII logger "header" options to "meta".

     - Fixes #763 "Escape # when first character in log file line".

All btests pass for me on Linux FC15. Will try MacOS next.

src/logging/WriterBackend.h

@@ -67,6 +67,11 @@ public:
 		 */
 		double rotation_base;
 
+		/**
+		 * The network time when the writer is created.
+		 */
+		double network_time;
+
 		/**
 		 * A map of key/value pairs corresponding to the relevant
 		 * filter's "config" table.
@@ -129,9 +134,11 @@ public:
 	 * Flushes any currently buffered output, assuming the writer
 	 * supports that. (If not, it will be ignored).
 	 *
+	 * @param network_time The network time when the flush was triggered.
+	 *
 	 * @return False if an error occured.
 	 */
-	bool Flush();
+	bool Flush(double network_time);
 
 	/**
 	 * Triggers rotation, if the writer supports that. (If not, it will
@@ -213,6 +220,10 @@ public:
 	  */
 	string Render(double d) const;
 
+	// Overridden from MsgThread.
+	virtual bool OnHeartbeat(double network_time, double current_time);
+	virtual bool OnFinish(double network_time);
+
 protected:
 	friend class FinishMessage;
 
@@ -272,8 +283,10 @@ protected:
 	 * will then be disabled and eventually deleted. When returning
 	 * false, an implementation should also call Error() to indicate what
 	 * happened.
+	 *
+	 * @param network_time The network time when the flush was triggered.
 	 */
-	virtual bool DoFlush() = 0;
+	virtual bool DoFlush(double network_time) = 0;
 
 	/**
 	 * Writer-specific method implementing log rotation.  Most directly
@@ -314,20 +327,19 @@ protected:
 
 	/**
 	 * Writer-specific method called just before the threading system is
-	 * going to shutdown.
+	 * going to shutdown. It is assumed that once this messages returns,
+	 * the thread can be safely terminated.
 	 *
-	 * This method can be overridden but one must call
-	 * WriterBackend::DoFinish().
+	 * @param network_time The network time when the finish is triggered.
 	 */
-	virtual bool DoFinish() { return MsgThread::DoFinish(); }
-
+	virtual bool DoFinish(double network_time) = 0;
 	/**
 	 * Triggered by regular heartbeat messages from the main thread.
 	 *
-	 * This method can be overridden but one must call
-	 * WriterBackend::DoHeartbeat().
+	 * This method can be overridden. Default implementation does
+	 * nothing.
 	 */
-	virtual bool DoHeartbeat(double network_time, double current_time);
+	virtual bool DoHeartbeat(double network_time, double current_time) = 0;
 
 private:
 	/**