http_parser: do not dealloc during kOnExecute

`freeParser` deallocates `Parser` instances early if they do not fit
into the free list. This does not play well with recent socket
consumption change, because it will try to deallocate the parser while
executing on its stack.

Regression was introduced in: 1bc44686

Fix: https://github.com/nodejs/node/issues/2928
PR-URL: https://github.com/nodejs/node/pull/2956


Reviewed-by: Trevor Norris <trev.norris@gmail.com>