TODO

* Restart server on the fly with some special command.
  - (reload-module (resolve-module '(ragnarok server))) 
  - (reload-extensions server)
    This step will refresh:
    1. env:mime-types-list;
    2. env:handler-list;
    3. the specified server's attributes;
    4. other servers won't change their behavior, but if they restart, new env works.

* Acctual logger operation should delay to the end of page handler. 
  That means client should get response as soon as possible. 

* Filter the target path, it should not appear '\n'

* Use catch/throw to handle the error
  + change error-page handler to catch-context

** deal with concurrent request
   Threads pool will be used in future version. It's not difficult but we just focus
   on the functional building now.

* deal with file uploading