Add MutableDocumentRevision and associated new Datastore methods:

    * createDocumentFromRevision(MutableDocumentRevision rev)
    * updateDocumentFromRevision(MutableDocumentRevision rev)
    * deleteDocumentFromRevision(DocumentRevision rev)
    * deleteDocument(String id)

Also update documentation including README.md and doc/crud.md
Allow mutable documents to be returned from ConflictResolver:

- This required a refactor of the DocumentRevision class hierarchy:

  - DocumentRevision becomes minimal parent interface of BasicDocumentRevision and
    MutableDocumentRevision

  - Almost all existing usages of DocumentRevision change to BasicDocumentRevision

- Also ensure that attachments are correctly set for the new winning revision

Also fix transactionality of attachment handling:

New prepareAttachments method makes it easier to prepare attachments and split out prepared and
already-saved attachments, before starting a transaction and committing them to the database.