Show time spent in request handler in access log

[clue]

This changeset adds the time spent in the request handler (middleware handlers + final controller) in the access log with millisecond precision.

2021-01-29 12:22:01.717 - "GET /users HTTP/1.1" 200 6 0.000

This is similar to the $request_time variable available in nginx access logs.

Builds on top of #45, #46, #47, #48
Refs reactphp/cache#35