Hierarchical logging

It would be nice to have a normalized log prefix all throughout the code:

Each component is present if applicable:

• cluster name: the name set through the API

• session id: a unique identifier (counter?), maybe the logged keyspace as well

• request id: a unique identifier (the CqlRequestHandler's hashCode should be good enough)

• execution index: 0 for the initial execution, 1-n for speculative executions. It already exists in the code.

• connection id: the remote address and port, maybe the local port as well

• stream id: the stream id when applicable

One challenging aspect is that we don't want this to break encapsulation, e.g. a connection should not have a reference to its session just to retrieve its id. We can solve that by passing an agnostic String logPrefix to constructors.