groonga: accept "/d/shutdown?mode=immediate" immediately even when all threads are used (#1076)
groonga: accept "/d/shutdown?mode=immediate" immediately even when all threads are used (#1076)

* groonga: modify to execute "shutdown --mode immediate" on the main thread.

Because if Groonga has not runnable thread when we execute
"shutdown --mode immediate", Groonga can not execute shutdown immediately.

* Use grn_expr_exec

* groonga: use grn_ctx_send

* Fix inverted condition

* Reuse HTTP header parser

Co-authored-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@cozmixng.org>

naoa/groonga,komainu8/groonga,naoa/groonga,groonga/groonga,naoa/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,naoa/groonga,kenhys/groonga,naoa/groonga,groonga/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,komainu8/groonga,komainu8/groonga,komainu8/groonga,kenhys/groonga,naoa/groonga,naoa/groonga,groonga/groonga