[WIP] Component dependency manager

[marcotc]

[The PR has not undergone any clean up. Naming of classes/modules are all up for change. File organization will be changed as well]

As part of the remote configuration effort, this PR changes the underlying component lifecycle management to use dependency injection.

This provides the following features:

• Components now declare all settings and components that they depend on.
  • This allows for a minimal and confident reconfiguration of components when settings are changed.
  • Components can additionally declare that they support "in-flight" reconfiguration (e.g. Sampling.sampling_rate=, instead of the default Sampling.new(sampling_rate)). This can reduce even further the overhead of reconfiguration.
  • It is not possible to create consistency component declaration: circular dependencies, forgetting to declare a dependency. All these error will be caught early in testing, when components are first loaded by ddtrace.
• Testing isolation is improved, as access to the global Datadog.configuration to retrieve settings is removed from components, and instead configuration is provided at initialization time.
• We don't have to initialize all components eagerly on initialization.
  • Full eager loading is still the behavior in this PR, to minimize risk, but this can be relaxed in the future.