Implement Git LFS basic-multipart transfer support

[shevron]

Following the addition in Giftless (datopian/giftless#11), we should add support for the new transfer mode in ckan-client-js so that it is possible to efficiently upload very large files.

Acceptance

• ckan-client-js can negotiate either a basic or multipart-basic transfer mode with a server that supports it
• ckan-client-js can decide whether to prefer a basic or multipart-basic transfer based on file size or other configuration
• ckan-client-js can successfully upload a large (>1gb) file to storage using mutlipart-basic

Technical Requirements

• Refactor current LFS logic to a separate module, with a single class wrapping the transfer logic and also encapsulating batch API / transfer mode negotiation
• Move basic transfer protocol implementation details into an adapter that can be replaced
• Implement a new multipart-basic transfer adapter