refactor: improves performance of the library

[jcchavezs]

This PR introduces some improvements in the library:

1. Introduces the onKernelResponse listener to have more accurate measurements of request latency.
2. Runs the flushTracer on the onKernelTerminate listener without the register_shutdown_function as this is the symfony idiomatic way to do expensive operations after serving the response (see https://symfony.com/doc/current/reference/events.html#kernel-terminate).
3. Retrieves the tracer and the extractor on construct instead of doing it every time a request comes in. This is no difference in PHP SAPI model but it is an improvement in the event loop model.

[jcchavezs]

Ping @mmoreram