MIDI: false warning

[Keith OHara]

This is the warning that Phil noticed in the regression-test comparison.  
In a situation where you typeset several files in one run with
   lilypond *.ly
such as four movements of a piece, the internal MIDI-channel counter might
exceed 16 and give a "warning: MIDI channel wrapped around". 

If each file uses 16-channels or fewer, then each MIDI file will play 
just fine.

% Instead of putting many input files on the command line, 
%  we can just request several MIDI files in a short test file
\score {<<
  \new Staff c
  \new Staff c
  \new Staff c
  \new Staff c >>
\midi {}  \midi {} \midi {} \midi {}
}

The simple input above demonstrates the false warning when 
midiChannelMapping=#'staff, the default since version 2.13.63.

When midiChannelMapping=#'instrument, the problem does not appear until 
the total number of unique instruments in all the files exceeds 16 (counting
all drumstaffs together as one).

It seems that Staff_performer::static_channel_map_ and channel_count_ should
be reset between MIDI output files.  Formerly, the channel counter was reset
in the hconstructor for "Performance".