Rename javadoc Messager class to JavadocLog

The name `Messager` as a class providing reporting facilities is an anachronism. The name is also confusingly similar to the name of the internal `Messages` class in the doclet world.

`Messager` is built on top of the javac `Log` class (it's a subtype), and there is a separate convention in the javadoc tool world of using a prefix of `Javadoc` for javadoc subtypes of javac classes ... e.g. JavadocEnter, JavadocClassFinder, etc.

With that in mind, it would make sense to rename `Messager` to `JavadocLog`.

Notes:
This only affects the internal "javadoc tool" world; doclets use the public `Reporter` interface.
Generally, the preferred capitalization of `javadoc` is `JavaDoc` ... but there is string/historical precedent in this specific context to use `Javadoc`.