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.

vfs/read_write_test.go

@@ -1,6 +1,7 @@
 package vfs
 
 import (
+	"context"
 	"io"
 	"io/ioutil"
 	"os"
@@ -25,7 +26,7 @@ func rwHandleCreateReadOnly(t *testing.T, r *fstest.Run) (*VFS, *RWFileHandle) {
 	opt.CacheMode = CacheModeFull
 	vfs := New(r.Fremote, &opt)
 
-	file1 := r.WriteObject("dir/file1", "0123456789abcdef", t1)
+	file1 := r.WriteObject(context.Background(), "dir/file1", "0123456789abcdef", t1)
 	fstest.CheckItems(t, r.Fremote, file1)
 
 	h, err := vfs.OpenFile("dir/file1", os.O_RDONLY, 0777)