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/FS.cpp

@@ -505,7 +505,19 @@ void File::append (const std::vector <std::string>& lines)
   {
     fseek (_fh, 0, SEEK_END);
     for (auto& line : lines)
-      fputs ((line + "\n").c_str (), _fh);
+      fputs (line.c_str (), _fh);
+  }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+void File::write_raw (const std::string& line)
+{
+  if (!_fh)
+    open ();
+
+  if (_fh)
+  {
+    fputs (line.c_str (), _fh);
   }
 }