Add API to handle server warnings

[whatyouhide]

Since native protocol v4, C* supports sending warnings in the responses that come from the server.

We should add an API that lets users of Xandra handle those warnings. The first proposal is an option to pass to Xandra.start_link/1, :warning_handler. It can be a function with two arguments or a {module, function, additional_args} tuple.

The function passed to this option would be called as:

case options[:warning_handler] do
  fun when is_function(fun, 2) -> fun.(query, warnings)
  {mod, fun, args} -> apply(mod, fun, [query, warnings | args])
end

Xandra should ship with a default implementation that uses Logger.warn/1. Basically, the default for :warning_handler would be something like

fn query, warnings ->
  Enum.each(warnings, fn warning -> 
    Logger.warn("Server warninng for query #{prettify(query)}: #{warning}")
  end)
end