Tokio support and multiplexing

[0xAlcibiades]

This PR updates the RedisSessionStore struct and its associated methods to use a redis::aio::ConnectionManager instead of a redis::Client. Using a multiplexed redis connection avoids a TCP connection/handshake from occurring on every session operation, yielding a minimum 25ms or so speedup even with a datacenter local redis instance.

Additionally, the PR adds a new method from_uri that allows creating a RedisSessionStore from a Redis URI instead of a redis::Client via from_client or from a ConnectionManager via new.

This also migrates the repo to the same runtime as the rest of the axum ecosystem. Resolving #21.

[0xAlcibiades]

Also resolves #20

[0xAlcibiades]

What is the use case for #15 with multiplexing/retry and error?

[0xAlcibiades]

@jbr do we want to add flags here for async-std/tokio?

[jbr]

@jbr do we want to add flags here for async-std/tokio?

yes, this crate should continue to support async-std as well as tokio