Support writing blocks to correct storage classes in Go SDK
Depends on #13382
The client is responsible for keeping track of how many replicas have been written for each storage class. If one storage class is satisfied but not another, the satisfied one can be removed from the X-Keep-Storage-Classes list. The overall write is a success if all storage classes were satisfied, and a failure if any of the storage classes could not be satisfied.
"We are not expecting to skip servers, we just need to send the storage classes and know when we are done"
17392-go-write-to-storage-class @ 757aa11abba0455c122430755a0c28b81f61ab36 -- https://ci.arvados.org/view/Developer/job/developer-run-tests/2406/