Upload Streaming
This MR adds upload streaming to siad with a new /renter/uploadstream
endpoint.
Right now there is no handling for interrupted uploads but a force
endpoint can be specified which causes an existing file to be deleted. A good follow-up would be to allow "repairing" an uploaded file by additionally specifying an offset to let siad know what offset within the file the start of the stream corresponds to.
Related to #3357 (closed)