Update sequoia-net to futures=0.2

Straightforward update to futures=0.2 with async/await. Some unrelated code changes due to running cargo fmt.

CI fails due to an old rust version, current stable does work locally.

Semi-related questions:

• It might be worth considering to migrate from failure to thiserror + anyhow, as failure is not really maintained and does not use std::error::Error which is cumbersome for users of sequoia. In addition, it is not clear to me if the custom Error enum is actually useful, as all methods return a Result<A, failure::Error>.
• Currently, all file system IO is done in a blocking manner. Should the "asyncified" versions in tokio::fs be used?
• The module docs in lib.rs still mention that a SKS keyserver is the default. Should I fix this in this MR?