Remove navigator.yieldForStorageUpdates() due to lack of implementation

[foolip]

Also rename NavigatorStorageUtils to NavigatorCookies.

Fixes #334

[annevk]

Would it perhaps be better to couple this with the removal of the storage mutex? Otherwise we might forget to add advice/warnings back.

[foolip]

What do you mean should be added back? AFAICT, the only overlap is in the "Serialisability of script execution" section, which should be fully removed or perhaps replaced with warnings, as noted in #335 (comment)

[annevk]

The "A call to X is implied" phrases seem like they might return in some way. E.g., if we adopt the model from @inexorabletash in that issue thread.

[foolip]

OK, so "A call to the navigator.yieldForStorageUpdates() method is implied when this method is invoked" corresponds to the "Release the storage mutex" step. It would be nice to remove those two things together, I suppose.

[foolip]

Not wanting to spend time untangling these issues, I'm doing both in #342