Support append and update for cached file systems

[AllanChain]

Fixes #1977

The main logic is:

• For r and a, try download first
• For simple read and no update, open a regular file handler
• For other cases, open a LocalTempFile

Some comments/questions:

• I'm confused at kwargs["mode"] = mode line, seems that we should always open compressed files using rb?
• I can't get open_many working for append. Seems the open_files passed to open_many is empty. This seems to be another issue.