plugin: don't increment reference count for temporary use
plugin: don't increment reference count for temporary use

Plugin registration isn't multi-thread safe. Should we add locks?

myokoym/groonga,hiroyuki-sato/groonga,myokoym/groonga,groonga/groonga,groonga/groonga,groonga/groonga,naoa/groonga,hiroyuki-sato/groonga,groonga/groonga,komainu8/groonga,naoa/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,myokoym/groonga,myokoym/groonga,naoa/groonga,naoa/groonga,hiroyuki-sato/groonga,hiroyuki-sato/groonga,redfigure/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,myokoym/groonga,naoa/groonga,cosmo0920/groonga,myokoym/groonga,redfigure/groonga,cosmo0920/groonga,cosmo0920/groonga,myokoym/groonga,hiroyuki-sato/groonga,kenhys/groonga,komainu8/groonga,redfigure/groonga,cosmo0920/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,myokoym/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,redfigure/groonga,naoa/groonga,redfigure/groonga,kenhys/groonga,cosmo0920/groonga,groonga/groonga,naoa/groonga,redfigure/groonga,cosmo0920/groonga,redfigure/groonga,komainu8/groonga,redfigure/groonga,kenhys/groonga,komainu8/groonga,kenhys/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,naoa/groonga