Dalli multiplexing

[delner]

This pull request adds multiplexing to Dalli instrumentation. Users can define configuration using the hostname port combo of the server to apply specific instrumentation settings:

Datadog.configure do |c|
  c.use :dalli, describes: 'localhost:1234', service_name: 'local-cache'
end

Then any Dalli client that sends to this hostname/port combo will have the trace settings applied, in this case commands sent to 'localhost:1234' will be tagged as the local-cache service. Commands that don't match this exactly will use default settings instead.

[marcotc]

If this is what we desire, the PR looks good!