Additional autosave speed improvement and 1 fix

This changes the autosave XML file to a binary representation
during writing to speed up autosave processing.  A lot of the
time used during autosave is a result of having to convert and
print all of the values to the XML file.

Writing the same information, but in binary format, reduces
all of that to just the bare essentials and the actual write
I/O.

During recovery, the binary file is read and converted to
the real xML representation and processing happens as it
did before.

It is a noticeable difference with very long or many tracks.

The included fix has to do with append recording.

Say you have 3 tracks and you want to append recorded audio
to the middle track.  Sometime later Audacity crashes and
upon recovery, the recorded audio is actually appended to
the third track, not the second one.

This fixes that by adding an "autosaveid" to each track as
it is written to the autosave file.  The same ID is written
to the recording recovery appends to the autosave file.

Then, during recovery, the IDs are matched up and the audio
gets appended to the proper track.

These autosaveid attributes are only present in the autosave
file and not in saved project files.

src/AudioIO.cpp

@@ -3384,7 +3384,7 @@ void AudioIO::FillBuffers()
       {
          // Append captured samples to the end of the WaveTracks.
          // The WaveTracks have their own buffering for efficiency.
-         XMLStringWriter blockFileLog;
+         AutoSaveFile blockFileLog;
          int numChannels = mCaptureTracks.GetCount();
 
          for( i = 0; (int)i < numChannels; i++ )
@@ -3392,7 +3392,7 @@ void AudioIO::FillBuffers()
             int avail = commonlyAvail;
             sampleFormat trackFormat = mCaptureTracks[i]->GetSampleFormat();
 
-            XMLStringWriter appendLog;
+            AutoSaveFile appendLog;
 
             if( mFactor == 1.0 )
             {
@@ -3423,6 +3423,7 @@ void AudioIO::FillBuffers()
             if (!appendLog.IsEmpty())
             {
                blockFileLog.StartTag(wxT("recordingrecovery"));
+               blockFileLog.WriteAttr(wxT("id"), mCaptureTracks[i]->GetAutoSaveIdent());
                blockFileLog.WriteAttr(wxT("channel"), (int)i);
                blockFileLog.WriteAttr(wxT("numchannels"), numChannels);
                blockFileLog.WriteSubTree(appendLog);