Fix waveform concurrency issues. Potentially fixes Bug #1383404.

* Use const shared pointers for waveforms.
* Move all std::vector sizing operations to the Waveform constructor. No
  vector state is allowed to change after the constructor finishes.
* Clarify comments.
* Remove waveform mutex usage in WOverview variants.
* Use the track's waveform pointer to atomically swap the waveform
  rather than mutating the waveform in place.
* Remove unused code.
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Merge pull request #420 from rryan/waveform

Fix waveform concurrency issues. Possible fix for Bug #1383404