Implement multipart support in Google Cloud storage backend

[shevron]

Following up on #11 and #51 (multipart on Azure), we should implement multipart for GCS as well.

Tasks

• [ ] A little bit research on Google Cloud's resumable uploads feature; Do not be confused by GCS's support for "multipart uploads" - this is unrelated and is related to uploading from a browser using multipart/form-data payload encoding.

• [ ] Implement MultipartStorage on GoogleCloudStorage

• [ ] Add tests (VCR) with mutlipart setup

Also, consider some refactoring of the boundary between transfer adapters and storage adapters, and fixing of "Verify" actions conflicting between Basic and Multipart transfers when both are enabled (and they should always both be enabled).