Unbundle SslContextFactory from jetty-util

Hi again,

This PR is more of a "is there any huge problem with this that I'm missing" rather than a "please merge this".

In Fedora, where I've just created an rpm package for this project, there are rules about bundled/forked stuff that we try to abide by where possible. We noticed during the initial package review, that the two classes removed here, came from jetty-util, so I've patched it with these changes to use the jetty classes.
Apart from missing the `io/tesla/aether/okhttp/ssl/` package, can you see any reason why this is a terrible idea? The missing package isn't a problem for us, since if there's another Fedora package that needs classes from this package, then they'll use the jetty ones also.

Of course, there are good reasons why you would probably not like to merge this PR, including:
- increased dependencies
- tests no longer pass: `npn-boot` test dep needs an older version of jetty-util (8.1.9.v20130131). There doesn't seem to be a later released `npn-boot` that uses the latest jetty-util. This isn't such a problem for our Fedora package, since we skip the tests anyway because of missing test dependencies.

Please let me know if there's something I'm overlooking here!

Thanks,
Gerard.