Token-aware routing (WIP)

[mpenick]

I started working on token-aware routing as a hobby project it's highly work-in-progress, but it's a start. This is one of three things that I think will make cql-proxy awesome for use with Apache Cassandra or any other CQL-based service as a general tool for handling various CQL routing related use cases:

• Token-aware routing (this PR)
• cql-proxy <--> backend cluster TLS (https://github.com/datastax/cql-proxy/pull/103)
• Pluggable authentication for client <--> proxy connections.

With these three changes cql-proxy could be used at the edge as way to terminate traffic for Cassandra deployed on k8s or as a way to implement lightweight drivers that don't have to implement token-awareness, or connection pooling, etc. Also, having the connection to Cassandra be persistent in the proxy makes it awesome for request-based platforms e.g. PHP in Apache (the HTTP server) because connecting to the local proxy is super fast and the slow connections to C* are pooled in the proxy. This could be useful for various other scripting languages and cron-based tasks too.

It's been a joy working on cql-proxy and I might check in on it time to time, but please take good care of the project!

[moguchev]

Will be helpful for ScyllaDB use