FS: Fix performance on high latency systems

fseek() in File::append() was invalidating the file buffer for each call
to append().  Better handling improves "import" performance test by 9%,
45% in "commit", on a system with a spinning disk.

This performance problem affects all operations where
{pending,completed}.data are rewritten.  During normal operation a
garbage collection can be enough to trigger it.  On storage with high
latency, e.g. networked, this previously took 20 seconds and more.

src/TDB2.cpp

@@ -257,14 +257,14 @@ void TF2::commit ()
           _file.lock ();
 
         // Write out all the added tasks.
+        _file.append (std::string(""));  // Seek to end of file
         for (auto& task : _added_tasks)
-          _file.append (task.composeF4 () + "\n");
+          _file.write_raw (task.composeF4 () + "\n");
 
         _added_tasks.clear ();
 
         // Write out all the added lines.
-        for (auto& line : _added_lines)
-          _file.append (line);
+        _file.append (_added_lines);
 
         _added_lines.clear ();
         _file.close ();
@@ -282,12 +282,12 @@ void TF2::commit ()
         _file.truncate ();
 
         // Only write out _tasks, because any deltas have already been applied.
+        _file.append (std::string(""));  // Seek to end of file
         for (auto& task : _tasks)
-          _file.append (task.composeF4 () + "\n");
+          _file.write_raw (task.composeF4 () + "\n");
 
         // Write out all the added lines.
-        for (auto& line : _added_lines)
-          _file.append (line);
+        _file.append (_added_lines);
 
         _added_lines.clear ();
         _file.close ();