Extract logger to components

[delner]

Based on #996 this pull request extracts the logger and debug mode to Datadog::Configuration::Components, consequently changing the old interface for configuring and using logging:

• Datadog::Logger.log --> Datadog.log
• Datadog::Logger.debug_logging --> Datadog.configuration.diagnostics.debug

And instead of:

logger = ::Logger(STDOUT)

Datadog.configure { |c| c.tracer log: logger, debug: true) }
# OR
Datadog.Logger.log = logger
Datadog.Logger.debug_logging = true

You configure with:

Datadog.configure do |c|
  c.logger = logger
  c.logger.level = ::Logger::INFO
  c.diagnostics.debug = true # Optionally forces logging to debug
end

Old style is still supported but will be deprecated in favor of this new style.

[marcotc]

One more giant leap towards better configuration 🥇

[marcotc]

Nice to see this test migrated to RSpec 💯