Rework request's logs on INFO level

[shindo]

Currently on INFO log level for each request the following will be printed:

• accepting connection from client
• access_log_entry on request finalization (the only log line with request ID)
• closing connection to client

What information could be helpful (on INFO log level):

• access_log_entry-like line after receiving headers from client (i.e. before request is processed)
  This should include received headers as well as it's part of the request (e.g., Apache Module mod_log_forensic).
• markers between keep-alive requests (process_next)
• in case of bad (bad request) or malformed (invalid url, missing handler) request additional information and context should be provided (such as headers, request ID, ...)
  This additional context not necessarily should be in the same log line, but it should be searchable by request ID.