Remove LegacyPromise in src/core/worker.js

Depends on PR #4725.

I wasn't sure what to make of https://github.com/mozilla/pdf.js/issues/4430#issue-29094940, so thus far this PR just replaces the Promises.

Another slight issue that I came across is the way that incorrect passwords are currently handled (see [worker.js#L256-L257](https://github.com/mozilla/pdf.js/blob/master/src/core/worker.js#L256-L257) and [pdf_manager.js#L70-L72](https://github.com/mozilla/pdf.js/blob/master/src/core/pdf_manager.js#L70-L72)). We basically reach into the `pdfManager` object and create a promise there, which seem all kinds of wrong :-)
I've attempted to rectify this situation, and the patch does work, but I'm not sure if this is the best solution. 

*I will squash the commits once this is ready to be merged.*