Added AvailableStreams function to Conn structure.

This exports the amount of Available Streams a connection has to user code. 

I am in need of this API as I am planning on creating a new host and connection pool in my gocql-contrib that will allow me to begin work on the host selection pipeline.

Specifically I am creating a connection pool for all the connections belonging to a host. I need this API so when I am picking a connection I can distribute queries to connections that are not maxed out.