This change updates the unit tests to have more comments so that it is
more clear what went wrong if a test fails.
Also, some utility code has been changed to make it easier to support
new platforms and read/write locks.
Also, the reference code has had some refactoring added to make it
easier for Widevine to test CDM code. There should be no
functionality differences in the reference code.
Also, in the main API doc, there was an obsolete paragraph in the
description of the threading model. This paragraph has been removed.
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