Better document content and lifetime management

[daveaglick]

After changing to a more parallel execution model in #486 combined with other issues like keeping too many open streams for file-based documents (#805), I think a total rework of this needs to be done before v3.

• Remove the DocumentFactory
• Remove the IExecutionContext.GetDocument() methods
• Instead, modules will directly instantiate documents
• Move Document to Wyam.Common for use as a base document type and make it abstract
• IDocument/Document should have methods to clone the document with new metadata (keeping the content stream, string, etc.)
• Create several specialized document types like StringDocument (which should continue to use the option of keeping content in memory or on disk), StreamDocument, and FileDocument - these are what modules should instantiate
• The IDocument.GetStream() method should lazily create the appropriate stream for the document (no more BlockingStream and other shenanigans, other than maybe for the StreamDocument)
• To support the old custom document type use case (#183), the user can register a decorator factory that will have a chance to wrap each document after each module (will need to think about how cloning works with a wrapper/decorator)
• The execution context will no longer track document creation and manage disposal life cycle. I need to think through the best way to manage disposal, which is really only needed for documents that have a stream which might need to be disposed (otherwise they can just be garbage collected). One thought is to use Document everywhere instead of IDocument which will let the base Document class do things like track instantiation and disposal.

[daveaglick]

After playing around with this, I think I've got a better idea that won't require quite as much infrastructure work:

• Because the document still needs to get a metadata instance with conversion super-powers from the context (and that's only available in Wyam.Core), we can't totally get rid of providing the context either in the constructor or with a factory using the context like we currently do. Might as well continue to use a factory.
• The underlying goal here is to specialize document content to streams, strings (in-memory or temporary files based on setting), and files and to better manage lifetime, particularly of content streams.
• Create a new IContentProvider interface and refactor all the GetDocument() and document factory methods/constructors to take that instead of a string or stream. It should have a IContentProvider.GetStream() method and be disposable (more on that below).
• The stream returned from the content provider should be disposed when done.
• Create implementations of the interface in Wyam.Common as StringContentProvider, StreamContentProvider, and FileContentProvider. Each implementation should have implicit conversion operators so you can just pass a IFile for example to the document factory methods.
• The context and documents should reference count content providers and only dispose them when no more documents are using it.

[daveaglick]

Done in the vnext branch.