groonga-httpd: extract logger set code
groonga-httpd: add missing return
groonga-httpd: reset logger per request
groonga-httpd: set loggers on closing database
doc: document cache_limit command

TODO:
  * update files
  * update po
doc: update file list

    % make -C doc update-files
doc: fix a typo

is ->
are
doc groonga-httpd: describe about proxy_cache parameter
doc: update po

    % make -C doc
doc: add documentation about experimental ruby_eval command
doc: update po

    % make -C doc/locale/ja update-po
doc: remove too much markers
doc cache_limit: translate
doc: add note and execution example about cascade delete
doc groonga-httpd: add more description about query log usage
doc: unify example log under "delete" directory
doc groonga-httpd: translate
doc: update file list
doc: update 'return value' section
doc: update po