Add context propagation to rclone

- Change rclone/fs interfaces to accept context.Context
- Update interface implementations to use context.Context
- Change top level usage to propagate context to lover level functions

Context propagation is needed for stopping transfers and passing other
request-scoped values.

cmd/copy/copy.go

@@ -1,6 +1,8 @@
 package copy
 
 import (
+	"context"
+
 	"github.com/ncw/rclone/cmd"
 	"github.com/ncw/rclone/fs/operations"
 	"github.com/ncw/rclone/fs/sync"
@@ -74,9 +76,9 @@ changed recently very efficiently like this:
 		fsrc, srcFileName, fdst := cmd.NewFsSrcFileDst(args)
 		cmd.Run(true, true, command, func() error {
 			if srcFileName == "" {
-				return sync.CopyDir(fdst, fsrc, createEmptySrcDirs)
+				return sync.CopyDir(context.Background(), fdst, fsrc, createEmptySrcDirs)
 			}
-			return operations.CopyFile(fdst, fsrc, srcFileName, srcFileName)
+			return operations.CopyFile(context.Background(), fdst, fsrc, srcFileName, srcFileName)
 		})
 	},
 }