Move client package to a new repository

[adutra]

I'd like to propose that we move the client package to a repository of its own.

The main reason is that this package is different from the others: it is not an implementation of protocol specs, but a small client-server couple to help testing protocol specs.

Also, it's gaining some popularity so it deserves to be better exposed and referenced.

And finally, it still needs some polishing before it could be considered production-ready, while the rest of this repository is already considered production-ready. Keeping the client package in this repository might lead users to mistakenly assume that this package is also production-ready.

[adutra]

The following are open issues related to the client package, which would need to be moved as well:

• Add support for secure bundle / SNI (Astra) #47
• Deadlock when opening and closing connections concurrently #38
• Panic when an in flight request is canceled #36

[adutra]

Also:

• CqlClientConnection.Close() blocked forever #55

[adutra]

The repository is created: https://github.com/datastax/go-cassandra-simple-client .