Migrating the TiKV Rust client from futures 0.1 to 0.3

I recently migrated a small/medium-sized crate from Futures 0.1 to 0.3. It was fairly easy, but there were some tricky bits and some things that were not well documented, so I think it is worth me writing up my experience. The crate I migrated is the Rust client for the TiKV database.