move temporary files from .dvc to .dvc/tmp

[efiop]

As requested by @dmpetrov in #3366 , having state db like that in the open might be badly perceived by the users lurking around their repo. We've seen people think badly about dvc because they think we use sqlite db as a core subsystem. Let's hide these files

• state
• updater
• updater.lock
• lock
  to .dvc/tmp.

[efiop]

Since we will be releasing 1.0 soon, might even do that in a non-backward compatible way to simplify the logic.