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.