Finalize bot model, rename Difficulty->Bot

[Terkwood]

Justification

Since the choice of bot depends on both timing and difficulty considerations, we renamed the Difficulty enum to Bot across the codebase.

At present only KataGoInstant and KataGoFullStrength are available as choices.

This naming can support the addition of GnuGo::Easy and GnuGo::Medium difficulties in #440, as well.

Additionally, we were able to remove a read call to Redis within botlink service by combining the concept of "is a bot attached to this game_id and player?" with the specific type of bot that was requested. This is locally named an Attachment -- and is not important to any other component in the system.

This ticket advances #322.

Work effort

Use rev 29163ac in Cargo.toml deps.

• DO NOT SQUASH the pull request (we're using a rev# in this PR for Cargo.toml deps)
• rebase commits
• rename and restructure Bot enum
• update botlink
• combine botlink repos
• update gateway
• update browser
• update tinybrain
• local test