Rework javadoc generation in bindings

This is a top-level tracker issue, which will need to be addressed in mdsal-binding-generator-impl and in mdsal-binding2-generator-impl.

The code here is twisted and inefficient, causing huge memory usage for strings during code generation, especially with large models.

Two things need to happen:

• YANG snippet needs to be generated and filtered as a CharSource, e.g. lazily, without instantiating the full thing before the class is actually output
• Snippet generation needs to work on top of org.opendaylight.yangtools.yang.model.api.meta.DeclaredStatement view of the world, so that 'uses' are not expanded, but rather the snippet quotes the actual YANG text