use async for read-only access to chunk server

Change src/client.rs to use async HTTP access to the chunk server for read-only operations. Everything but obnam backup to make a new backup is now async. Each subcommand uses block_on to call and wait for an async function. When all subcommands are async, we can make main be async.

I ran into problems changing the backup code to also be async. The insert_iter couldn't handle async results. Will need to think about how to refactor the code and architecture for that part of the client, but wanted to get this out for review now, rather than wait.

I will merge this on Monday, unless there are objections. I will continue work on asyncifying the backup code after this is merged.